Correcting pyramidal error of polygon scanner in scanning beam display systems

ABSTRACT

Scanning beam display systems using fluorescent screens and various servo feedback control mechanisms to control display imaging qualities, including techniques and mechanism for measuring and correcting pyramidal errors of a polygon scanner.

This application claims the benefits of the following U.S. ProvisionalPatent Applications

1. Ser. No. 60/773,993 entitled “Display Systems Using OpticalFluorescent Screens and Servo Feedback Control for Such Systems” andfiled on Feb. 15, 2006;

2. Ser. No. 60/776,553 entitled “Pyramidal Error Correction in LaserDisplays Using Polygon Scanners” and filed on Feb. 24, 2006;

3. Ser. No. 60/779,261 entitled “Display Systems Using Scanning Lightand Electronic Correction of Optical Distortion by Imaging LensAssembly” and filed on Mar. 3, 2006; and

4. Ser. No. 60/800,870 entitled “Display Systems Using FluorescentScreens Including Fluorescent Screens With Prismatic Layer” and filed onMay 15, 2006.

This application claims the benefit of PCT patent application No.PCT/US2006/11757 entitled “Display Systems Having Screens With OpticalFluorescent Materials” and filed Mar. 31, 2006.

In addition, this application is a continuation-in-part application ofand claims the benefits of U.S. application Ser. No. 11/515,420 entitled“Servo-Assisted Scanning Beam Display Systems Using Fluorescent Screens”and filed Sep. 1, 2006, which claims the benefits of the U.S.Provisional Patent Applications Nos. 1, 3 and 4 listed above and theabove referenced PCT application.

This application incorporates by reference the entire disclosures of theabove referenced patent applications as part of the specification ofthis application.

BACKGROUND

This application relates to scanning-beam display systems includingsystems that use polygon scanners to scan one or more optical beams onscreens.

In a scanning-beam display system, an optical beam can be scanned over ascreen to form images on the screen. Many display systems such as laserdisplay systems use a polygon scanner with multiple reflective facets toprovide horizontal scanning and a vertical scanning mirror such as agalvo-driven mirror to provide vertical scanning. In operation, onefacet of the polygon scanner scans one horizontal line as the polygonscanner spins to change the orientation and position of the facet andthe next facet scans the next horizontal line. The horizontal scanningand the vertical scanning are synchronized to each other to projectimages on the screen.

Such scanning-beam display systems can be in various configurations. Forexample, scanning-beam display systems may use passive screens that donot emit light but make light of the scanning beam visible to a viewerby one or a combination of mechanisms, such as optical reflection,optical diffusion, optical scattering and optical diffraction. Variousfront and rear projection displays use passive screens. Scanning-beamdisplay systems can also use active screens such as fluorescent screensthat include fluorescent materials to emit colored light under opticalexcitation where the emitted light forms the images visible to viewers.

SUMMARY

The specification of this application describes, among others, displaysystems and devices based on scanning light on a screen. Multiple laserscan be used to simultaneously scan multiple laser beams to illuminateone screen. For example, the multiple laser beams can illuminate onescreen segment at a time and sequentially scan multiple screen segmentsto complete a full screen. The screen can include fluorescent materialswhich emit visible light under excitation of the scanning light to formimages with the emitted visible light.

In one implementation, a scanning beam display system is described toinclude an optical module operable to produce a scanning beam ofexcitation light having optical pulses that are sequential in time andcarry image information; a fluorescent screen which absorbs theexcitation light and emits visible fluorescent light to produce imagescarried by the scanning beam; and an optical sensor positioned toreceive a feedback optical signal generated by the fluorescent screenunder illumination of the scanning beam and to produce a monitor signalindicating a spatial alignment of the optical pulses on the fluorescentscreen. The optical module comprises a feedback control unit operable toadjust timing of the optical pulses carried by the scanning beam inresponse to the monitor signal to control the spatial alignment ofspatial positions of the optical pulses on the fluorescent screen.

In the above scanning beam display system, the screen can includeparallel fluorescent stripes which produce the images carried by thescanning beam, and servo reference marks respectively located atboundaries of the fluorescent stripes to produce the feedback opticalsignal under illumination of the scanning beam. The feedback opticalsignal varies in amplitude with a position of the scanning beam acrosseach fluorescent stripe, and the optical module is operable to create atemporal variation in timing of the optical pulses in the scanning beamto shift positions of the optical pulses on the screen along a beamscanning direction perpendicular to the fluorescent stripes. Inaddition, the feedback control unit is operable to adjust timing of theoptical pulses in response to information in the monitor signal todirect a position of each optical pulse towards a center of afluorescent stripe along the beam scanning direction.

In another implementation, a method for controlling a scanning beamdisplay system is described. In this method, a beam of excitation lightmodulated with optical pulses is scanned on a screen with parallelfluorescent stripes in a beam scanning direction perpendicular to thefluorescent stripes to excite the fluorescent strips to emit visiblefluorescent light which forms images. A temporal variation in timing ofthe optical pulses in the beam of excitation light is provided toadvance or delay a spatial position of each optical pulse along the beamscanning direction on the screen. A reflection of the beam of excitationlight from the screen is detected to produce a monitor signal whoseamplitude varies with a position of the beam relative to a fluorescentstripe. Next, the monitor signal is processed to obtain information on aspatial offset of a position of an optical pulse on the screen relativeto a center of a fluorescent stripe and the timing of the optical pulsesin the beam of excitation light is adjusted to reduce the spatialoffset.

In the above method, the following operations may be conducted tofurther control the system. A peripheral servo reference mark can beprovided outside the fluorescent stripes in the beam scanning directionto produce a feedback light when illuminated by the scanning beam. Thescanning beam is then controlled to scan over the peripheral servoreference mark during a scan over the fluorescent area. The scanningbeam is controlled to be in a CW mode when the scanning beam is scanningover the peripheral servo reference mark and to be in a pulsed mode tocarry the optical pulses when the scanning beam is scanning over thefluorescent stripes. The feedback light from the peripheral servoreference mark is used to detect a beam parameter of the scanning beamand the detected beam parameter is used to adjust the scanning beam. Theperipheral servo reference mark may be used to achieve various controls,such as beam focusing, vertical beam position on the screen, and thebeam power on the screen.

In yet another implementation, a scanning beam display system caninclude an optical module operable to produce a scanning beam ofexcitation light having optical pulses that are sequential in time andcarry image information, and a fluorescent screen that includes afluorescent area and a peripheral servo reference mark area outside thefluorescent area. The fluorescent area absorbs the excitation light andemits visible fluorescent light to produce images carried by thescanning beam. The fluorescent area includes first servo reference markswhich produce a first feedback optical signal under illumination of thescanning beam. The peripheral servo reference mark area includes atleast one second servo reference mark that produces a second feedbackoptical signal under illumination of the scanning beam. This system alsoincludes a first optical sensor positioned to receive the first feedbackoptical signal and to produce a first monitor signal indicating aspatial alignment of the optical pulses on the fluorescent screen, and asecond optical sensor positioned to receive the second feedback opticalsignal and to produce a second monitor signal indicating an opticalproperty of the scanning beam on the fluorescent screen. The opticalmodule includes a feedback control unit to adjust the scanning beam inresponse to the first and second monitor signals to control at least thespatial alignment of spatial positions of the optical pulses on thefluorescent screen.

The screen in the above system may further include a light pipe formedin the peripheral servo reference mark area of the screen. This lightpipe has an input portion that is coupled to receive the second feedbackoptical signal generated by the second servo reference mark and anoutput portion that is coupled to the second optical sensor to directthe received second feedback optical signal to the second opticalsensor. The second servo reference mark may be optically transmissive todirect a transmitted portion of the scanning beam to the light pipe asthe second feedback optical signal.

Examples of scanning beam display systems with a first scanner and asecond polygon scanner are described. In example, such a system includean optical module and a screen. The optical module includes a firstscanner to scan along a first direction at least one scanning beamhaving optical pulses that are sequential in time and carry imageinformation, and a second scanner having a polygon with reflectivefacets. The polygon is operable to rotate around a rotation axis that isalong the first direction to scan the at least one scanning beam along asecond direction perpendicular to the first direction. The screen ispositioned to receive the at least one scanning beam from the opticalmodule and configured to include (1) a display region which displaysimages carried by the at least one scanning beam, and (2) referencemarks positioned in paths along the second direction of the least onescanning beam on the screen and displaced from one another along thefirst direction. Each reference mark is operable to produce an opticalmonitor signal when illuminated by the at least one scanning beam. Thesystem also includes an optical detector positioned to receive theoptical monitor signal from the screen and to produce a detector signalcontaining information on a position offset of the least one scanningbeam relative to a respective reference mark on the screen, and a firstscanner control that measures a pyramidal error of the polygon from thedetector signal and controls scanning of the second scanner to correctthe position offset caused by the pyramidal error.

A method for operating a scanning beam display system with two scannersfor scanning along two directions is also described. This methodincludes using a first scanner to scan at least one beam of lightmodulated with optical pulses to carry images along a first direction ona screen and a second polygon scanner with reflective facets to scan theat least one beam along a second, perpendicular direction on the screento display the images. Reference marks on the screen at positions thatare respectively in beam scanning paths of the at least one beam by thereflective facets, respectively, are used to produce optical monitorsignals when illuminated by the at least one beam during scanning. Eachoptical monitor signal has information on a position offset of the leastone beam relative to a respective reference mark on the screen caused bya pyramidal error of a respective reflective facet in the polygonscanner. This method further includes detecting the optical monitorsignals from the screen to produce a detector signal containing theinformation on the position offset; and adjusting the scanning of thefirst scanner along the first direction to reduce the position offset ofthe at least one beam on the screen in response to the position offsetin the detector signal.

Another example of a scanning beam display system with two scannersincludes an optical module operable to produce a scanning beam ofexcitation light having optical pulses that are sequential in time andcarry image information, a first scanner to scan the scanning beam alonga first direction, a second scanner comprising a polygon havingreflective facets and operable to spin around an axis parallel to thefirst direction and to use the reflective facets to scan the scanningbeam along a second, perpendicular direction, and a fluorescent screencomprising a fluorescent area having parallel fluorescent stripes eachlong the first direction and spatially displaced from one another alongthe second direction and a peripheral servo reference mark area outsidethe fluorescent area. The fluorescent stripes absorb the excitationlight and emit visible fluorescent light to produce images carried bythe scanning beam. The fluorescent area also includes first servoreference marks producing a first feedback optical signal underillumination of the scanning beam to indicate a spatial alignment of theoptical pulses to the fluorescent stripes along the second direction.The peripheral servo reference mark area includes second servo referencemarks each producing a second feedback optical signal under illuminationof the scanning beam indicating a position offset of the scanning beamalong the first direction. This system also includes a first opticalsensor positioned to receive the first feedback optical signal and toproduce a first monitor signal indicating the spatial alignment of theoptical pulses relative to the fluorescent stripes, a second opticalsensor positioned to receive the second feedback optical signal and toproduce a second monitor signal indicating the position offset of thescanning beam along the first direction when scanned by a respectivereflective facet, and a control unit operable to adjust the scanningbeam in response to the first and second monitor signals to control atleast the spatial alignment of spatial positions of the optical pulsesrelative to the fluorescent stripes and to reduce the position offset ofthe scanning beam along the first direction.

This application also describes an example of a scanning beam displaysystem with two scanners that includes a polygon scanner havingreflector facets and operable to rotate to scan an optical beam along afirst direction, a second scanner having a reflector to cause theoptical beam to scan in a second direction perpendicular to the firstdirection, and a control unit in communication with the second scannerto control scanning of the second scanner. The control unit is operableto dither the second scanner to cause the optical beam to change itsdirection back and forth along the second direction during each scanningat a dither frequency higher than a frame rate of an image carried bythe optical beam.

In addition, this application describes an example of a method fordisplay using two scanners that uses a polygon scanner having reflectorfacets to scan an optical beam along a first direction and uses a secondscanner having a reflector to scan the optical beam in a seconddirection perpendicular to the first direction. This method alsoincludes controlling the scanning of the optical beam to scan theoptical beam with different facets of the polygon scanner at eachhorizontal scanning line in successive frames.

These and other examples and implementations are described in detail inthe drawings, the detailed description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example scanning laser display system having afluorescent screen made of laser-excitable fluorescent materials (e.g.,phosphors) emitting colored lights under excitation of a scanning laserbeam that carries the image information to be displayed.

FIGS. 2A and 2B show one example screen structure with parallelfluorescent stripes and the structure of color pixels on the screen inFIG. 1.

FIG. 2C shows another example for a fluorescent screen with fluorescentstripes formed by placing parallel optical filters over the layer of auniform fluorescent layer which emits white light under opticalexcitation.

FIGS. 3 and 4 show two different scanning beam displays.

FIG. 5 shows an example implementation of the laser module in FIG. 3having multiple lasers that direct multiple laser beams on the screen.

FIG. 6 shows one example for time division on each modulated laser beam120 where each color pixel time is equally divided into three sequentialtime slots for the three color channels.

FIG. 7 shows one example for simultaneously scanning consecutive scanlines with multiple excitation laser beams.

FIG. 8 shows one example of a scanning display system using a servofeedback control and an on-screen optical sensing unit.

FIG. 9 shows one example of a fluorescent screen with on-screen opticalservo detectors.

FIG. 10 shows one example of a scanning display system using a servofeedback control and an off-screen optical sensing unit.

FIG. 11 shows an example of a fluorescent screen having peripheralreference mark regions that include servo reference marks that producefeedback light for various servo control functions.

FIG. 12 shows a start of line reference mark in a peripheral referencemark region to provide a reference for the beginning of the activefluorescent area on the screen.

FIG. 13 shows an example of a vertical beam position reference mark forthe screen in FIG. 11.

FIGS. 14A and 14B show a servo feedback control circuit and itsoperation in using the vertical beam position reference mark in FIG. 13to control the vertical beam position on the screen.

FIGS. 15 and 16 show another example of a vertical beam positionreference mark for the screen in FIG. 11 and a corresponding servofeedback control circuit.

FIG. 17 shows an example of a laser actuator that controls the verticaldirection of the laser beam for the servo control of the vertical beamposition on the screen.

FIG. 18 shows an example of a beam focus sensing mark for the screen inFIG. 11 to provide a servo feedback for controlling the beam focus onthe screen.

FIG. 19 shows one implementation of the screen in FIG. 11 that includesvarious reference marks including a power sensing mark for monitoringthe optical power of the excitation beam on the screen

FIGS. 20A, 20B, 20C and 20D illustrate an operation of the servofeedback control in scanning display system in FIG. 8 based on detectinga test pattern for red, green and blue colors.

FIG. 21 shows one example of a scanning display system with servofeedback control based on servo reference marks on the screen and atemporal variation on the timing of the optical pulses in the excitationbeam.

FIGS. 22, 23 and 24 show examples of fluorescent screens having servoreference marks that produce feedback light for the servo control.

FIG. 25 shows timing of optical pulses and beam positions on afluorescent screen with fluorescent stripes.

FIGS. 26A, 26B and 26C illustrate operations of servo reference marks onstripe dividers in a fluorescent screen when the pulse is turned on atdifferent beam positions along the horizontal scan directionperpendicular to the fluorescent stripes.

FIG. 27 shows spatial dependency of reflected excitation signals byservo reference marks on stripe dividers in a fluorescent screen.

FIG. 28 illustrates three regions within a subpixel that have threedifferent power levels for the reflected excitation signals, where servoreference marks are formed on stripe dividers.

FIGS. 29, 30, 31 and 32 illustrate operations of the servo referencemarks formed on stripe dividers in response to a periodic temporal delaysignal on the timing of the optical pulses in the excitation beam.

FIG. 33 illustrates generation of error signals from the reflectedsignals from the servo reference marks on stripe dividers based on theperiodic temporal delay signal on the timing of the optical pulses inthe excitation beam shown in FIGS. 20, 30, 31 and 32.

FIGS. 34, 35 and 36 illustrate examples of calibrating a fluorescentscreen by scanning the screen in a CW mode to obtain measurements of adetected reflected feedback light as a function of the scan time for aportion of one horizontal scan, the respective output of the peakdetector and the sampling clock signal.

FIG. 37 illustrates scanning of the vertical scanner (e.g., the galvomirror) in the scanning display shown in FIG. 5.

FIGS. 38, 39A and 39B illustrates an effect of a pyramidal error of thepolygon scanner on the beam position on the screen.

FIG. 40 illustrates a dithering operation of the vertical scanner in thescanning display in FIG. 5.

FIGS. 41 and 42 illustrate use of vertical reference marks in aperipheral region of the screen to detect pyramidal errors of facets ofthe polygon scanner in a scanning display system.

FIG. 43 shows an example of a scanning beam display system thatimplements a pyramidal error monitor mechanism and a pyramidal errorcorrection mechanism.

FIG. 44 shows correction of pyramidal errors in display one video framein an example system based on the design in FIG. 43.

DETAILED DESCRIPTION

Examples of scanning-beam display systems described in this applicationuse a vertical scanning mirror and a rotating polygon mirror to providethe 2-dimensional scanning of one or more scanning beams on the screento form images. A beam may be first directed to the vertical scanningmirror and then to the horizontal polygon mirror or in a reverse order.In operation, in tracing a horizontal line by scanning the polygonscanner, the vertical scanning mirror operates to displace thehorizontal lines vertically. The vertical scanning mirror can beimplemented by, e.g., using a mirror engaged to a galvanometer as thevertical scanner.

Different mirror facets on the polygon mirror may not be exactly at thesame orientation with respect to the rotation axis of the polygonscanner (e.g., the vertical direction) and thus different facets maydirect the same beam at different vertical directions. This deviationfrom one facet to another facet is known as the pyramidal error and cancause errors in vertical positions of different horizontal lines scannedby different facets of the polygon scanner. This pyramidal error candegrade the image quality on the screen. When a polygon is free of thepyramidal error, multiple horizontal lines on the screen scanned bydifferent facets are equally spaced if the vertical scanning mirroroperates at a constant scanning speed in the vertical direction. If thepolygon scanner, however, has the pyramidal error, the horizontal lineson the screen from different facets are not equally spaced when thevertical scanner operates at a constant scanning speed in the verticaldirection. The variation in the line spacing between two adjacenthorizontal scan lines depends on the difference in orientations of therespective adjacent facets of the polygon scanner. Such uneven linespacing can distort the displayed images, and degrade the image qualitysuch as colors, resolution, and other quality factors of the imagesdisplayed on the screen.

A polygon scanner can be designed and manufactured with a high precisionto minimize the pyramidal error. Polygons with low pyramidal errors,however, can be expensive. To reduce the cost, a pyramidal errorcorrection mechanism can be implemented in such a system to correct theknown pyramidal errors of an installed polygon scanner. Implementationof this correction mechanism allows the use of relatively inexpensivepolygons with pyramidal errors without compromising the displayperformance. In addition, the orientations of facets of a polygonscanner may change with time due to various factors, such as a change intemperature and other environmental factors (e.g., humidity), aging ofthe materials used in a polygon scanner over time, and others.Furthermore, a polygon scanner in a system may be replaced by adifferent polygon scanner due to malfunction or failure of the originalpolygon and such replacement can change the pyramidal errors because twodifferent polygons tend to have different pyramidal errors. Hence, tomaintain a high image quality in presence of variations of pyramidalerrors, the pyramidal error correction mechanism can be designed toprovide adjustable corrections to the pyramidal errors as the pyramidalerrors of facets change.

This application describes examples of techniques and correctionmechanisms for pyramidal error correction and other aspects ofscanning-beam displays. The described techniques and correctionmechanisms for pyramidal error correction can be implemented inscanning-beam displays with both “passive” screens and active screens. Apassive screen does not emit light but makes light of the one or morescanning beams visible to a viewer by one or a combination ofmechanisms, such as optical reflection, optical diffusion, opticalscattering and optical diffraction. For example, a passive screen canreflect or scatter received scanning beam(s) to show images. An activescreen emits light by absorbing the one or more scanning beams and theemitted light forms part of or all of the light that forms the displayedimages. Such an active screen may include one or more fluorescentmaterials to emit light under optical excitation of the one or morescanning beams received by the screen to produce images. Screens withphosphor materials under excitation of one or more scanning excitationlaser beams are described here as specific implementation examples ofoptically excited fluorescent materials in various system.

The following sections first describe examples of scanning beam displaysystems and devices that use fluorescent screens with fluorescentmaterials to emit light under optical excitation to produce images andthen describe techniques and mechanisms for pyramidal error correctionused in scanning-beam display systems using either passive screens oractive screens.

Scanning-beam display systems using fluorescent screens can includelaser vector scanner display devices and laser video display devicesthat use laser excitable fluorescent screens to produce images byabsorbing excitation laser light and emitting colored light. Variousexamples of screen designs with fluorescent materials are described.Screens with phosphor materials under excitation of one or more scanningexcitation laser beams are described in details and are used as specificimplementation examples of optically excited fluorescent materials invarious system and device examples in this application. In oneimplementation, for example, three different color phosphors that areoptically excitable by the laser beam to respectively produce light inred, green, and blue colors suitable for forming color images can beformed on the screen as repetitive red, green and blue phosphor stripesin parallel. Various examples described in this application use screenswith parallel color phosphor stripes for emitting light in red, green,and blue to illustrate various features of the laser-based displays.Phosphor materials are one type of fluorescent materials. Variousdescribed systems, devices and features in the examples that usephosphors as the fluorescent materials are applicable to displays withscreens made of other optically excitable, light-emitting, non-phosphorfluorescent materials.

For example, quantum dot materials emit light under proper opticalexcitation and thus can be used as the fluorescent materials for systemsand devices in this application. More specifically, semiconductorcompounds such as, among others, CdSe and PbS, can be fabricated in formof particles with a diameter on the order of the exciton Bohr radius ofthe compounds as quantum dot materials to emit light. To produce lightof different colors, different quantum dot materials with differentenergy band gap structures may be used to emit different colors underthe same excitation light. Some quantum dots are between 2 and 10nanometers in size and include approximately tens of atoms such between10 to 50 atoms. Quantum dots may be dispersed and mixed in variousmaterials to form liquid solutions, powders, jelly-like matrix materialsand solids (e.g., solid solutions). Quantum dot films or film stripesmay be formed on a substrate as a screen for a system or device in thisapplication. In one implementation, for example, three different quantumdot materials can be designed and engineered to be optically excited bythe scanning laser beam as the optical pump to produce light in red,green, and blue colors suitable for forming color images. Such quantumdots may be formed on the screen as pixel dots arranged in parallellines (e.g., repetitive sequential red pixel dot line, green pixel dotline and blue pixel dot line).

Some implementations of laser-based display techniques and systemsdescribed here use at least one scanning laser beam to excite colorlight-emitting materials deposited on a screen to produce color images.The scanning laser beam is modulated to carry images in red, green andblue colors or in other visible colors and is controlled in such a waythat the laser beam excites the color light-emitting materials in red,green and blue colors with images in red, green and blue colors,respectively. Hence, the scanning laser beam carries the images but doesnot directly produce the visible light seen by a viewer. Instead, thecolor light-emitting fluorescent materials on the screen absorb theenergy of the scanning laser beam and emit visible light in red, greenand blue or other colors to generate actual color images seen by theviewer.

Laser excitation of the fluorescent materials using one or more laserbeams with energy sufficient to cause the fluorescent materials to emitlight or to luminance is one of various forms of optical excitation. Inother implementations, the optical excitation may be generated by anon-laser light source that is sufficiently energetic to excite thefluorescent materials used in the screen. Examples of non-laserexcitation light sources include various light-emitting diodes (LEDs),light lamps and other light sources that produce light at a wavelengthor a spectral band to excite a fluorescent material that converts thelight of a higher energy into light of lower energy in the visiblerange. The excitation optical beam that excites a fluorescent materialon the screen can be at a frequency or in a spectral range that ishigher in frequency than the frequency of the emitted visible light bythe fluorescent material. Accordingly, the excitation optical beam maybe in the violet spectral range and the ultra violet (UV) spectralrange, e.g., wavelengths under 420 nm. In the examples described below,UV light or a UV laser beam is used as an example of the excitationlight for a phosphor material or other fluorescent material and may belight at other wavelength.

FIG. 1 illustrates an example of a laser-based display system using ascreen having color phosphor stripes. Alternatively, color phosphor dotsmay also be used to define the image pixels on the screen. The systemincludes a laser module 110 to produce and project at least one scanninglaser beam 120 onto a screen 101. The screen 101 has parallel colorphosphor stripes in the vertical direction where red phosphor absorbsthe laser light to emit light in red, green phosphor absorbs the laserlight to emit light in green and blue phosphor absorbs the laser lightto emit light in blue. Adjacent three color phosphor stripes are inthree different colors. One particular spatial color sequence of thestripes is shown in FIG. 1 as red, green and blue. Other color sequencesmay also be used. The laser beam 120 is at the wavelength within theoptical absorption bandwidth of the color phosphors and is usually at awavelength shorter than the visible blue and the green and red colorsfor the color images. As an example, the color phosphors may bephosphors that absorb UV light in the spectral range from about 380 nmto about 420 nm to produce desired red, green and blue light. The lasermodule 110 can include one or more lasers such as UV diode lasers toproduce the beam 120, a beam scanning mechanism to scan the beam 120horizontally and vertically to render one image frame at a time on thescreen 101, and a signal modulation mechanism to modulate the beam 120to carry the information for image channels for red, green and bluecolors. Such display systems may be configured as rear scanner systemswhere the viewer and the laser module 110 are on the opposite sides ofthe screen 101. Alternatively, such display systems may be configured asfront scanner systems where the viewer and laser module 110 are on thesame side of the screen 101.

FIG. 2A shows an exemplary design of the screen 101 in FIG. 1. Thescreen 101 in this particular example includes a rear substrate 201which is transparent to the scanning laser beam 120 and faces the lasermodule 110 to receive the scanning laser beam 120. A second frontsubstrate 202 is fixed relative to the rear substrate 201 and faces theviewer so that the fluorescent light transmits through the substrate 202towards the viewer. A color phosphor stripe layer 203 is placed betweenthe substrates 201 and 202 and includes phosphor stripes. The colorphosphor stripes for emitting red, green and blue colors are representedby “R”, “G” and “B,” respectively. The front substrate 202 istransparent to the red, green and blue colors emitted by the phosphorstripes. The substrates 201 and 202 may be made of various materials,including glass or plastic panels. Each color pixel includes portions ofthree adjacent color phosphor stripes in the horizontal direction andits vertical dimension is defined by the beam spread of the laser beam120 in the vertical direction. As such, each color pixel includes threesubpixels of three different colors (e.g., the red, green and blue). Thelaser module 110 scans the laser beam 120 one horizontal line at a time,e.g., from left to right and from top to bottom to fill the screen 101.The laser module 110 is fixed in position relative to the screen 101 sothat the scanning of the beam 120 can be controlled in a predeterminedmanner to ensure proper alignment between the laser beam 120 and eachpixel position on the screen 101.

In FIG. 2A, the scanning laser beam 120 is directed at the greenphosphor stripe within a pixel to produce green light for that pixel.FIG. 2B further shows the operation of the screen 101 in a view alongthe direction perpendicular to the surface of the screen 101. Since eachcolor stripe is longitudinal in shape, the cross section of the beam 120may be shaped to be elongated along the direction of the stripe tomaximize the fill factor of the beam within each color stripe for apixel. This may be achieved by using a beam shaping optical element inthe laser module 110. A laser source that is used to produce a scanninglaser beam that excites a phosphor material on the screen may be asingle mode laser or a multimode laser. The laser may also be a singlemode along the direction perpendicular to the elongated directionphosphor stripes to have a small beam spread that is confined by thewidth of each phosphor stripe. Along the elongated direction of thephosphor stripes, this laser beam may have multiple modes to spread overa larger area than the beam spread in the direction across the phosphorstripe. This use of a laser beam with a single mode in one direction tohave a small beam footprint on the screen and multiple modes in theperpendicular direction to have a larger footprint on the screen allowsthe beam to be shaped to fit the elongated color subpixel on the screenand to provide sufficient laser power in the beam via the multimodes toensure sufficient brightness of the screen.

Alternatively, FIG. 2C illustrates an example of a fluorescent screendesign that has a contiguous and uniform layer 220 of mixed phosphors.This mixed phosphor layer 220 is designed and constructed to emit whitelight under optical excitation of the excitation light 120. The mixedphosphors in the mixed phosphor layer 220 can be designed in variousways and a number of compositions for the mixed phosphors that emitwhite light are known and documented. Notably, a layer 210 of colorfilters, such as stripes of red-transmitting, green-transmitting andblue-transmitting filters, is placed on the viewer side of the mixedphosphor layer 220 to filter the white light and to produce coloredoutput light. The layers 210 and 220 can be sandwiched betweensubstrates 201 and 202. The color filters may be implemented in variousconfigurations, including in designs similar to the color filters usedin color LCD panels. In each color filter region e.g., ared-transmitting filter, the filter transmits the red light and absorbslight of other colors including green light and blue light. Each filterin the layer 210 may be a multi-layer structure that effectuates aband-pass interference filter with a desired transmission band. Variousdesigns and techniques may be used for designing and constructing suchfilters. U.S. Pat. No. 5,587,818 entitled “Three color LCD with a blackmatrix and red and/or blue filters on one substrate and with greenfilters and red and/or blue filters on the opposite substrate,” and U.S.Pat. No. 5,684,552 entitled “Color liquid crystal display having a colorfilter composed of multilayer thin films,” for example, describe red,green and blue filters that may be used in the screen design in FIG. 2C.Hence, a fluorescent stripe in the fluorescent screen 101 in variousexamples described in this application is a fluorescent stripe thatemits a designated color under optical excitation and can be either afluorescent stripe formed of a particular fluorescent material thatemits the designed color in FIG. 2A or a combination of a stripe colorfilter and a white fluorescent layer in FIG. 2C.

The optical modulation in the laser module 110 in FIG. 1 can beimplemented in two different configurations. FIG. 3 shows animplementation of the display in FIG. 1 where a laser source 310 such asa diode laser is directly modulated to produce a modulated excitationbeam 312 that carries the image signals in red, green and blue. Thelaser module 110 in this implementation includes a signal modulationcontroller 320 which modulates the laser source 310 directly. Forexample, the signal modulation controller 320 can control the drivingcurrent of a laser diode as the laser source 310. A beam scanning andimaging module 330 then scans and projects the modulated excitation beam312 as the scanning excitation beam 120 to the screen 101 to excite thecolor phosphors.

Alternatively, FIG. 4 shows another implementation of the display inFIG. 1 where a laser source 410 is used to generate a CW unmodulatedexcitation laser beam 412 and an optical modulator 420 is used tomodulate the CW excitation laser beam 412 with the image signals in red,green and blue and to produce a modulated excitation beam 422. A signalmodulation controller 430 is used to control the optical modulator 420.For example, an acousto-optic modulator or an electro-optic modulatormay be used as the optical modulator 420. The modulated beam 422 fromthe optical modulator 420 is then scanned and projected onto the screen101 by the beam scanning and imaging module 330 as the scanningexcitation beam 120.

FIG. 5 shows an example implementation of the laser module 110 inFIG. 1. A laser array 510 with multiple lasers is used to generatemultiple laser beams 512 to simultaneously scan the screen 101 forenhanced display brightness. The laser array 510 can be implemented invarious configurations, such as discrete laser diodes on separate chipsarranged in an array and a monolithic laser array chip having integratedlaser diodes arranged in an array. A signal modulation controller 520 isprovided to control and modulate the lasers in the laser array 510 sothat the laser beams 512 are modulated to carry the image to bedisplayed on the screen 101. The signal modulation controller 520 caninclude a digital image processor which generate the digital imagesignals for the three different color channels and laser driver circuitsthat produce laser control signals carrying the digital image signals.The laser control signals are then applied to modulate the lasers in thelaser array 510, e.g., the currents for laser diodes.

The beam scanning is achieved by using a scanning mirror 540 such as agalvo mirror for the vertical scanning and a multi-facet polygon scanner550 for the horizontal scanning. A scan lens 560 is used to project thescanning beams form the polygon scanner 550 onto the screen 101. Thescan lens 560 is designed to image each laser in the laser array 510onto the screen 101. Each of the different reflective facets of thepolygon scanner 550 simultaneously scans N horizontal lines where N isthe number of lasers. In the illustrated example, the laser beams arefirst directed to the galvo mirror 540 and then from the galvo mirror540 to the polygon scanner 550. The output scanning beams 120 are thenprojected onto the screen 101. A relay optics module 530 is placed inthe optical path of the laser beams 512 to modify the spatial propertyof the laser beams 512 and to produce a closely packed bundle of beams532 for scanning by the galvo mirror 540 and the polygon scanner 550 asthe scanning beams 520 projected onto the screen 101 to excite thephosphors and to generate the images by colored light emitted by thephosphors.

The laser beams 120 are scanned spatially across the screen 101 to hitdifferent color pixels at different times. Accordingly, each of themodulated beams 120 carries the image signals for the red, green andblue colors for each pixel at different times and for different pixelsat different times. Hence, the beams 120 are coded with imageinformation for different pixels at different times by the signalmodulation controller 520. The beam scanning thus maps the timely codedimage signals in the beams 120 onto the spatial pixels on the screen101.

For example, FIG. 6 shows one example for time division on eachmodulated laser beam 120 where each color pixel time is equally dividedinto three sequential time slots for the three color channels. Themodulation of the beam 120 may use pulse modulation techniques, such aspulse width modulation, pulse amplitude modulation or a combination ofpulse width modulation and pulse amplitude modulation, to producedesired grey scales in each color, proper color combination in eachpixel, and desired image brightness.

The beams 120 on the screen 101 are located at different and adjacentvertical positions with two adjacent beams being spaced from each otheron the screen 101 by one horizontal line of the screen 101 along thevertical direction. For a given position of the galvo mirror 540 and agiven position of the polygon scanner 550, the beams 120 may not bealigned with each other along the vertical direction on the screen 101and may be at different positions on the screen 101 along the horizontaldirection. The beams 120 can cover one portion of the screen 101. At afixed angular position of the galvo mirror 540, the spinning of thepolygon scanner 550 causes the beams 120 from N lasers in the laserarray 510 to scan one screen segment of N adjacent horizontal lines onthe screen 101. At the end of each horizontal scan, the galvo mirror 540is adjusted to a different fixed angular position so that the verticalpositions of all N beams 120 are adjusted to scan the next adjacentscreen segment of N horizontal lines. This process iterates until theentire screen 101 is scanned to produce a full screen display.

FIG. 7 illustrates the above simultaneous scanning of one screen segmentwith multiple scanning laser beams 120 at a time. Visually, the beams120 behaves like a paint brush to “paint” one thick horizontal strokeacross the screen 101 at a time to cover one screen segment between thestart edge and the end edge of the image area of the screen 101 and thensubsequently to “paint” another thick horizontal stroke to cover anadjacent vertically shifted screen segment. Assuming the laser array 310has 36 lasers, a 1080-line progressive scan of the screen 101 wouldrequire scanning 30 vertical screen segments for a full scan. Hence,this configuration in an effect divides the screen 101 along thevertical direction into multiple screen segments so that the N scanningbeams scan one screen segment at a time with each scanning beam scanningonly one line in the screen segment and different beams scanningdifferent sequential lines in that screen segment. After one screensegment is scanned, the N scanning beams are moved at the same time toscan the next adjacent screen segment.

In the above design with multiple laser beams, each scanning laser beam120 scans only a number of lines across the entire screen along thevertical direction that is equal to the number of screen segments.Hence, the polygon scanner 550 for the horizontal scanning can operateat slower speeds than scanning speeds required for a single beam designwhere the single beam scans every line of the entire screen. For a givennumber of total horizontal lines on the screen (e.g., 1080 lines inHDTV), the number of screen segments decreases as the number of thelasers increases. Hence, with 36 lasers, the galvo mirror and thepolygon scanner scan 30 lines per frame while a total of 108 lines perframe are scanned when there are only 10 lasers. Therefore, the use ofthe multiple lasers can increase the image brightness which isapproximately proportional to the number of lasers used, and, at thesame time, can also advantageously reduce the response speeds of thescanning system.

A scanning display system described in this specification can becalibrated during the manufacture process so that the laser beam on-offtiming and position of the laser beam relative to the fluorescentstripes in the screen 101 are known and are controlled within apermissible tolerance margin in order for the system to properly operatewith specified image quality. However, the screen 101 and components inthe laser module 101 of the system can change over time due to variousfactors, such as scanning device jitter, changes in temperature orhumidity, changes in orientation of the system relative to gravity,settling due to vibration, aging and others. Such changes can affect thepositioning of the laser source relative to the screen 101 over time andthus the factory-set alignment can be altered due to such changes.Notably, such changes can produce visible and, often undesirable,effects on the displayed images. For example, a laser pulse in thescanning excitation beam 120 may hit a subpixel that is adjacent to anintended target subpixel for that laser pulse due to a misalignment ofthe scanning beam 120 relative to the screen along the horizontalscanning direction. When this occurs, the coloring of the displayedimage is changed from the intended coloring of the image. Hence, a redflag in the intended image may be displayed as a green flag on thescreen. For another example, a laser pulse in the scanning excitationbeam 120 may hit both the intended target subpixel and an adjacentsubpixel next to the intended target subpixel due to a misalignment ofthe scanning beam 120 relative to the screen along the horizontalscanning direction. When this occurs, the coloring of the displayedimage is changed from the intended coloring of the image and the imageresolution deteriorates. The visible effects of these changes canincrease as the screen display resolution increases because a smallerpixel means a smaller tolerance for a change in position. In addition,as the size of the screen increases, the effect of a change that canaffect the alignment can be more pronounced because a large moment armassociated with a large screen means that an angular error can lead to alarge position error on the screen. For example, if the laser beamposition on the screen for a known beam angle changes over time, theresult is a color shift in the image. This effect can be noticeable andthus undesirable to the viewer.

Implementations of various alignment mechanisms are provided in thisspecification to maintain proper alignment of the scanning beam 120 onthe desired sub-pixel to achieved desired image quality. These alignmentmechanisms include reference marks on the screen, both in thefluorescent area and in one or more peripheral area outside thefluorescent area, to provide feedback light that is caused by theexcitation beam 120 and represents the position and other properties ofthe scanning beam on the screen. The feedback light can be measured byusing one or more optical servo sensors to produce a feedback servosignal. A servo control in the laser module 110 processes this feedbackservo signal to extract the information on the beam positioning andother properties of the beam on the screen and, in response, adjust thedirection and other properties of the scanning beam 120 to ensure theproper operation of the display system.

For example, a feedback servo control system can be provided to useperipheral servo reference marks positioned outside the display areaunobservable by the viewer to provide control over various beamproperties, such as the horizontal positioning along the horizontalscanning direction perpendicular to the fluorescent stripes, thevertical positioning along the longitudinal direction of the fluorescentstripes, the beam focusing on the screen for control the imagesharpness, and the beam power on the screen for control the imagebrightness. For another example, a screen calibration procedure can beperformed at the startup of the display system to measure the beamposition information as a calibration map so having the exact positionsof sub-pixels on the screen in the time domain. This calibration map isthen used by the laser module 110 to control the timing and positioningof the scanning beam 120 to achieve the desired color purity. For yetanother example, a dynamic servo control system can be provided toregularly update the calibration map during the normal operation of thedisplay system by using servo reference marks in the fluorescent area ofthe screen to provide the feedback light without affecting the viewingexperience of a viewer.

The following sections first describe examples of screen detectiontechniques and servo feedback implementations.

Two optical detection methods can be used to detect the location of abeam relative to a target feature on the screen, which may be a subpixelor a selected position on the screen such as the beginning edge of thefluorescent area. In the first optical detection method, the lightimpinging on a servo reference mark for the target feature can be guidedas the feedback light through air or other medium to reach one or morerespective optical servo sensing detectors which convert the opticallight levels of the feedback light into electrical amplitude signals.The second optical detection method uses one or more optical servosensing detectors placed in air to collect diffused light from a servoreference mark on the screen as the feedback light for the servocontrol. In detecting diffused light, an optical servo sensing detectorcan be placed behind a collection lens such as a hemispherical lens.Radiation detectors can be used to detect feedback light from diffusivetype targets, e.g., targets that allow the light to diffuse in a wideangular spectrum. An example of a diffuse target is a rough surface suchas a surface with a white paint. Both techniques can be used withreflective or transmissive servo reference marks.

FIG. 8 shows an exemplary scanning beam display system with an on-screenoptical sensing unit and a feedback control to allow the laser module110 to correct the horizontal misalignment. The screen 101 includes anon-screen optical sensing unit 810 for optically measuring the responsesof color subpixels on the screen 101 to produce a sensor feedback signal812. The laser module 110 has a feedback control to allow the lasermodule 110 to correct the misalignment in response to the feedbacksignal 812 from the screen 101.

FIG. 9 shows one example of the on-screen optical sensing unit 810 whichincludes three optical “direct” detectors PD1, PD2 and PD3 that arerespectively configured to respond to red, green and blue light. In thisspecific example, three beam splitters BS1, BS2 and BS3 are placedbehind red, green and blue subpixels of a color pixel, respectively andare used to split small fractions of red, green, and blue light beamsemitted from the color sub pixels of the color pixel to the threedetectors PD1, PD2 and PD3 formed on the front substrate of the screen101. Alternatively, the above red, green and blue optical detectors PD1,PD2 and PD3 may also be positioned on the screen 101 to allow eachdetector to receive light from multiple pixels on the screen 101. Eachoptical detector is only responsive to its designated color to produce acorresponding detector output and does not produce a detector outputwhen receiving light of other colors. Hence, the red optical detectorPD1 detects only the red light and is not responsive to green and bluelight; the green optical detector PD 2 detects only green light and isnot responsive to red and blue light; and the blue optical detector PD3detects only the blue light and is not responsive to red and greenlight. This color selective response of the one-screen optical sensingunit 810 may be achieved by, e.g., using red, green and blue opticalbandpass filters in front of the optical detectors PD1, PD2 and PD3,respectively, when each detector is exposed to light of different colorsfrom the screen 101, or placing the optical detectors PD1, PD2 and PD3in a way that only light of a designated color can enter a respectiveoptical detector for the designated color. Assume the adjacent colorphosphor stripes are arranged in the order of red, green and blue fromthe left to the right in the horizontal direction of the screen 101.Consider a situation where a red image is generated by the displayprocessor in the laser module 110. When the horizontal alignment is outof order or misaligned by one sub pixel, the red detector does notrespond while either the blue detector or the green detector produces anoutput. Such detector outputs can be processed by the feedback controlin the laser module 110 to detect the horizontal misalignment and,accordingly, can adjust the timing of the optical pulses in the scanningbeam to correct misalignment.

Alternative to the beam splitter in FIG. 9 a light guide or light pipecan be used. Light guides are structures that guide a portion of thelight to an optical servo sensing detector. A light guide can be formedon the screen to direct feedback light via total internal reflection(TIR) in the light guide to the detector.

FIG. 10 shows another scanning beam display system with a servo feedbackcontrol using a radiation style detector. In this system, an off-screenoptical sensing unit 1010 is used to detect the red, green and bluelight emitted from the screen. Three optical detectors PD1, PD2 and PD3are provided in the sensing unit 1010 to detect the red, green and bluefluorescent light, respectively. Each optical detector is designed toreceive light from a part of or the entire screen. A bandpass opticalfilter can be placed in front of each optical detector to select adesignated color while rejecting light of other colors.

For the screen 101, additional alignment reference marks can be used todetermine the relative position of the beam and the screen and otherparameters of the excitation beam on the screen. For example, during ahorizontal scan of the excitation beam 120 across the fluorescentstripes, a start of line mark can be provided for the system todetermine the beginning of the active fluorescent display area of thescreen 101 so that the signal modulation controller of the system canbegin deliver optical pulses to the targeted pixels. An end of line markcan also be provided for the system to determine the end of the activefluorescent display area of the screen 101 during a horizontal scan. Foranother example, a vertical alignment referenced mark can be providedfor the system to determine whether the beam 120 is pointed to a propervertical location on the screen. Other examples for reference marks maybe one or more reference marks for measuring the beam spot size on thescreen and one or more reference marks on the screen to measure theoptical power of the excitation beam 120. Such reference marks can beplaced a region outside the active fluorescent area of the screen 101,e.g., in one or more peripheral regions of the active fluorescent screenarea.

FIG. 11 illustrates one example of a fluorescent screen 101 havingperipheral reference mark regions. The screen 101 includes a centralactive fluorescent area 2600 with parallel fluorescent stripes fordisplaying images, two stripe peripheral reference mark regions 2610 and2620 that are parallel to the fluorescent stripes. Each peripheralreference mark region can be used to provide various reference marks forthe screen 101. In some implementations, only the left peripheralreference mark region 2610 is provided without the second region 2620when the horizontal scan across the fluorescent stripes is directed fromthe left to the right of the area 2600.

Such a peripheral reference mark region on the screen 101 allows thescanning display system to monitor certain operating parameters of thesystem. Notably, because a reference mark in the peripheral referencemark region is outside the active fluorescent display area 2600 of thescreen 101, a corresponding servo feedback control function can beperformed outside the duration during the display operation when theexcitation beam is scanning through the active fluorescent display area2600 to display image. Therefore, a dynamic servo operation can beimplemented without interfering the display of the images to the viewer.In this regard, each scan can include a CW mode period when anexcitation beam sans through the peripheral referenced mark region forthe dynamic servo sensing and control and a display mode period when themodulation of the excitation beam is turned on to produce image-carryingoptical pulses as the excitation beam sans through the activefluorescent display area 2600.

FIG. 12 shows an example of a start of line (SOL) reference mark 2710 inthe left peripheral region 2610 in the screen 101. The SOL referencemark 2710 can be an optically reflective, diffusive or fluorescentstripe parallel to the fluorescent stripes in the active fluorescentregion 2600 of the screen 101. The SOL reference mark 2710 is fixed at aposition with a known distance from the first fluorescent stripe in theregion 2600. SOL patterns may include multiple vertical lines withuniform or variable spacing. Multiple lines are selected for redundancy,increasing signal to noise, accuracy of position (time) measurement, andproviding missing pulse detection.

In operation, the scanning excitation beam 120 is scanned from the leftto the right in the screen 101 by first scanning through the peripheralreference mark region 2610 and then through the active fluorescentregion 2600. When the beam 120 is in the peripheral reference markregion 2610, the signal modulation controller in the laser module 110 ofthe system sets the beam 120 in a CW mode without the modulated opticalpulses that carry the image data. When the scanning excitation beam 120scans through the SOL reference mark 2710, the light reflected,scattered or emitted by the SOL reference mark 2710 due to theillumination by the excitation beam 2710 can be measured at an SOLoptical detector located near the SOL reference mark 2710. The presenceof this signal indicates the location of the beam 120. The SOL opticaldetector can be fixed at a location in the region 2610 on the screen 101or off the screen 101. Therefore, the SOL reference mark 2710 can beused to allow for periodic alignment adjustment during the lifetime ofthe system.

The laser beam is turned on continuously as a CW beam before the beamreaches the SOL mark 2710 in a scan. When the pulse from the SOLdetected is detected, the laser can be controlled to operate in theimage mode and carry optical pulses with imaging data. The system thenrecalls a previously measured value for the delay from SOL pulse tobeginning of the image area. This process can be implemented in eachhorizontal scan to ensure that each line starts the image area properlyaligned to the color stripes. The correction is made prior to paintingthe image for that line, so there is no lag in correction allowing forboth high frequency (up to line scan rate) and low frequency errors tobe corrected.

Physical implementation of the SOL sensor may be a reflective (specularor diffuse) pattern with an area detector(s), an aperture mask withlight pipe to collect the transmitted light into a single detector ormultiple detectors.

With reflective method, multiple lasers on and passing over reflectiveareas simultaneously may create self interference. A method to preventthis is to space the laser beams such that only one active beam passesover the reflective area at a time. It is likely that some reflectionwill come from the image area of the screen. To prevent this frominterfering with the SOL sensor signal, the active laser beams may bespaced such that no other laser beams are active over any reflectivearea when the desired active laser beam is passing over the reflectiveSOL sensor area. The transmission method is not affected by reflectionsfrom the image area.

Similar to the SOL mark 2710, an end-of-line (EOL) reference mark can beimplemented on the opposite side of the screen 101, e.g., in theperipheral reference mark region 2620 in FIG. 11. The SOL mark is usedto ensure the proper alignment of the laser beam with the beginning ofthe image area. This does not ensure the proper alignment during theentire horizontal scan because the position errors can be present acrossthe screen. Implementing the EOL reference mark and an end-of-lineoptical detector in the region 2620 can be used to provide a linear, twopoint correction of laser beam position across the image area.

When both SOL and EOL marks are implemented, the laser is turned oncontinuously in a continuous wave (CW) mode prior to reaching the EOLsensor area. Once the EOL signal is detected, the laser can be returnedto image mode and timing (or scan speed) correction calculations aremade based on the time difference between the SOL and EOL pulses. Thesecorrections are applied to the next one or more lines. Multiple lines ofSOL to EOL time measurements can be averaged to reduce noise.

In addition to control of the horizontal beam position along the scandirection perpendicular to the fluorescent stripes, the beam positionalong the vertical position parallel to the fluorescent stripes can alsobe monitored and controlled to ensure the image quality. Referring toFIG. 2B, each fluorescent stripe may not have any physical boundariesbetween two pixels along the vertical direction. This is different fromthe pixilation along the horizontal scan direction perpendicular to thefluorescent stripes. The pixel positions along the fluorescent stripesare controlled by the vertical beam position on the screen to ensure aconstant and uniform vertical pixel positions without overlapping andgap between two different horizontal scan lines. Referring to themulti-beam scanning configuration in FIG. 7, when multiple excitationbeams are used to simultaneously scan consecutive horizontal scan withinone screen segment on the screen, the proper vertical alignment of thelasers to one another are important to ensure a uniform vertical spacingbetween two adjacent laser beams on the screen and to ensure a propervertical alignment between two adjacent screen segments along thevertical direction. In addition, the vertical positioning information onthe screen can be used to provide feedback to control the verticalscanner amplitude and measure the linearity of the vertical scanner.

Vertical position of each laser can be adjusted by using an actuator, avertical scanner such as the galvo mirror 540 in FIG. 5, an adjustablelens in the optical path of each laser beam or a combination of theseand other mechanisms. Vertical reference marks can be provided on thescreen to allow for a vertical servo feedback from the screen to thelaser module. One or more reflective, fluorescent or transmissivevertical reference marks can be provided adjacent to the image area ofthe screen 101 to measure the vertical position of each excitation beam120. Referring to FIG. 11, such vertical reference marks can be placedin a peripheral reference mark region. One or more vertical mark opticaldetectors can be used to measure the reflected, fluorescent ortransmitted light from a vertical reference mark when illuminated by theexcitation beam 120. The output of each vertical mark optical detectoris processed and the information on the beam vertical position is usedto control an actuator to adjust the vertical beam position on thescreen 101.

FIG. 13 shows an example of a vertical reference mark 2810. The mark2810 includes is a pair of identical triangle reference marks 2811 and2812 that are separated and spaced from each other in both vertical andhorizontal directions to maintain an overlap along the horizontaldirection. Each triangle reference mark 2811 or 2812 is oriented tocreate a variation in the area along the vertical direction so that thebeam 120 partially overlaps with each mark when scanning through themark along the horizontal direction. As the vertical position of thebeam 120 changes, the overlapping area on the mark with the beam 120changes in size. The relative positions of the two marks 2811 and 2812defines a predetermined vertical beam position and the scanning beamalong a horizontal line across this predetermined vertical positionscans through the equal areas as indicated by the shadowed areas in thetwo marks 2811 and 2812. When the beam position is above thispredetermined vertical beam position, the beam sees a bigger mark areain the first mark 2811 than the mark area in the second mark 2812 andthis difference in the mark areas seen by the beam increases as the beamposition moves further up along the vertical direction. Conversely, whenthe beam position is below this predetermined vertical beam position,the beam sees a bigger mark area in the second mark 2812 than the markarea in the first mark 2811 and this difference in the mark areas seenby the beam increases as the beam position moves further down along thevertical direction.

The feedback light from each triangle mark is integrated over the markand the integrated signals of the two marks are compared to produce adifferential signal. The sign of the differential signal indicated thedirection of the offset from the predetermined vertical beam positionand the magnitude of the differential signal indicates the amount of theoffset. The excitation beam is at the proper vertical position when theintegrated light from each triangle is equal, i.e., the differentialsignal is zero.

FIG. 14A shows a portion of the signal processing circuit as part of thevertical beam position servo feedback control in the laser module 110for the vertical reference mark in FIG. 13. A PIN diode preamplifier2910 receives and amplifies the differential signal for the tworeflected signals from the two marks 2811 and 2812 and directs theamplified differential signal to an integrator 2920. Ananalog-to-digital converter 2930 is provided to convert the differentialsignal into a digital signal. A digital processor 2940 processes thedifferential signal to determine the amount and direction of theadjustment in the vertical beam position and accordingly produces avertical actuator control signal. This control signal is converted intoan analog control signal by a digital to analog converter 2950 and isapplied to a vertical actuator controller 2960 which adjusts theactuator. FIG. 14B further shows generation of the differential signalby using a single optical detector.

FIG. 15 shows another example of a vertical reference mark 3010 and aportion of the signal processing in a servo control circuit in FIG. 16.The mark 3010 includes a pair of reference marks 3011 and 3012 that areseparated and spaced from each other in the horizontal scan directionand the horizontal distance DX(Y) between the two marks 3011 and 3012 isa monotonic function of the vertical beam position Y. The first mark3011 can be a vertical stripe and the second mark 3012 can be a stripeat a slanted angle from the vertical direction. For a given horizontalscanning speed on the screen, the time for the beam to scan from thefirst mark 3011 to the second mark 3022 is a function of the verticalbeam position. For a predetermined vertical beam position, thecorresponding scan time for the beam to scan through the two marks 3011and 3012 is a fixed scan time. One or two optical detectors can be usedto detect the reflected light from the two marks 3011 and 3012 and thetwo optical pulses or peaks reflected by the two marks for theexcitation beam 120 in the CW mode can be measured to determine the timeinterval between the two optical pulses. The difference between themeasured scan time and the fixed scan time for the predeterminedvertical beam position can be used to determine the offset and thedirection of the offset in the vertical beam position. A feedbackcontrol signal is then applied to the vertical, actuator to reduce thevertical offset.

FIG. 16 shows a portion of the signal processing circuit as part of thevertical beam position servo feedback control in the laser module 110for the vertical reference mark in FIG. 15. A PIN diode preamplifier3110 receives and amplifies the detector output signal from an opticaldetector that detects the reflected light from the two marks 3011 and3012 during a horizontal scan. The amplified signal is processed by apulse detector 3120 to produce corresponding pulses corresponding to thetwo optical pulses at different times in the reflected light. A timeinterval measurement circuit 3130 is used to measure the time betweenthe two pulses and this time measurement is converted into a digitalsignal in a analog to digital converter 3140 for processing by a digitalprocessor 3150. The digital processor 3150 determines the amount anddirection of the an adjustment in the vertical beam position based onthe measured time and accordingly produces a vertical actuator controlsignal. This control signal is converted into an analog control signalby a digital to analog converter 3160 and is applied to a verticalactuator controller 2960 which adjusts the actuator.

A vertical reference mark may also be implemented by using a singletriangular reference mark shown in FIG. 13 where the single trianglereference mark 2811 or 2812 is oriented to create a variation in thehorizontal dimension of the mark along the vertical direction so thatthe beam 120 partially overlaps with the mark when scanning through themark along the horizontal direction. When the vertical position of thebeam 120 changes, the horizontal width of the mark scanned by the beam120 changes. Hence, when the beam 120 scans over the mark, an opticalpulse is generated in the reflected or fluorescent light generated bythe mark and the width of the generated optical pulse is proportional tothe horizontal width of the mark which is a function of the verticalbeam position. At a predetermined vertical beam position, the opticalpulse width is a fixed value. Therefore, this fixed optical pulse widthcan be used as a reference to determine the vertical position of thebeam 120 relative to the predetermined vertical beam position based onthe difference between the optical pulse width associated with thescanning of the beam 120 across the mark. An optical detector can beplaced near the mark to detector the reflected or fluorescent light fromthe mark and the difference in the width of the pulse from the fixedvalue can be used to as a feedback control to adjust the verticalactuator for the beam 120 to reduce the offset of the vertical beamposition.

In implementing multiple lasers for simultaneously scanning consecutivelines within one of multiple screen segments as shown in FIG. 7, twoseparate vertical positioning servo control mechanisms can beimplemented. The first vertical positioning servo control is to controlthe line to line spacing of different horizontal lines scanned bydifferent lasers at the same time within each screen segment.Accordingly, at each line, a vertical reference mark and an associatedoptical detector are needed to provide servo feedback to control thevertical beam position of each laser beam. Hence, this first verticalservo control mechanism includes N vertical servo feedback controls forthe N lasers, respectively.

The second vertical positioning servo control is to control the verticalalignment between two adjacent screen segments by using the galvo mirror540 in FIG. 5 to vertically move all N laser beams, after completion ofscanning one screen segment, to an adjacent screen segment. This can beachieved by controlling the galvo mirror 540 to make a common adjustmentin the vertical direction for all N laser beams. The vertical referencemark in the peripheral reference mark region 2610 in FIG. 11 and theassociated optical detector for the top line in each screen segment canbe used to measure the vertical position of the first of the N laserbeams when the beams are still scanning through the peripheral referencemark region 2610 in FIG. 11. This vertical information obtained in thismeasurement is used as a feedback signal to control the vertical angleof the galvo mirror 540 to correct any vertical error indicated in themeasurement. In implementations, this correction can lead to a smallamplitude (micro-jog) correction signal to the vertical galvo 540 forthat scan line.

The vertical alignment between two adjacent screen segments isdetermined by a number of factors, including the galvo linearity atdifferent galvo angles of the galvo mirror 540, the polygon pyramidalerrors of the polygon scanner 550, and optical system distortions causedby various reflective and refractive optical elements such as mirrorsand lenses. The polygon pyramidal errors are errors in the vertical beampositions caused by different tilting angles in the vertical directionat different polygon facets of the polygon 550 due to the manufacturingtolerance. One manufacturing tolerance on the polygon mirror is thepyramidal error of the facets. The implementation of the second verticalpositioning servo control can compensate for the polygon pyramidalerrors and thus a relatively inexpensive polygon scanner can be used inthe present scanning display systems without significantly compromisingthe display quality.

The second vertical servo control based on the galvo micro-jogcorrection signal can also use a look-up table of pyramidal error valuesof the polygon 550. The pyramidal errors in this look-up table can beobtained from prior measurements. When a pyramidal error does not changesignificantly with temperature, humidity and others, this look-up tablemethod may be sufficient without using the servo feedback based on ameasured vertical beam position using the vertical reference markdescribed above. In implementation, the feedback control needs theidentification of the polygon facet that is currently scanning a lineand thus can retrieve the corresponding pyramidal error value for thatpolygon facet from the look-up table. The identification of the currentpolygon facet can be determined from a facet number sensor on thepolygon 550.

In the above vertical servo feedback control for each individual laser,a laser actuator is used to adjust the vertical direction of the laserbeam in response to the servo feedback and to place the beam at adesired vertical beam position along a fluorescent stripe on the screen.FIG. 17 shows one example of a laser actuator 3240 engaged to acollimator lens 3230 which is placed in front of a laser diode 3210 tocollimate the laser beam produced by the laser 3210. The collimated beamout of the collimator lens 3230 is scanned by the galvo mirror 540 andthe polygon scanner 550 and is projected on the screen 101 by the scanlens 560. The laser diode 3210, the collimator lens 3230 and the lensactuator 3240 are mounted on a laser mount 3220. The lens actuator 3240can adjust the vertical position of the collimator lens 3230 along thevertical direction that is substantially perpendicular to the laserbeam. This adjustment of the collimator lens 3230 changes the verticaldirection of the laser beam and thus the vertical beam position on thescreen 101. The lens actuator 3240 may also move the position of thecollimator lens 3230 along the propagation direction of the laser beamand thus the focusing of the collimator lens 3230 on the laser beam.This adjustment can change the beam spot size on the screen 101.

The beam spot size for each excitation beam 120 on the screen 101 needsto be controlled to be less than each subpixel size to achieve thedesired display resolution and color purity. If the beam spot size islarger than each subpixel, a portion of the beam can spill over into anadjacent fluorescent stripe to excite one or two wrong colors and reducethe amount of fluorescent light emitted in that subpixel. These effectscan degrade the image quality such as the image resolution and colorsaturation. The focusing of a scanning excitation beam in a scanningdisplay system can set an optimal focusing condition at the factory.This factory focusing setting, however, can change due to variations intemperature and other factors. Therefore, a beam focusing servo controlcan be implemented to maintain the proper beam focusing.

FIG. 18 illustrate a focus sensing mark 3310 located in a peripheralreference mark region 2610 or 2620 on the screen 101. The focus sensingmark 3310 can be optically reflective, fluorescent or transmissive tomeasure the beam spot size. An optical detector can be placed near thefocus sensing mark 3310 to produce a detector signal that carries thebeam spot size information. A servo feedback control can be used toadjust the focusing of the laser beam on the screen 101. For example,the collimator lens 3230 can be adjusted by using the lens actuator 3240in FIG. 17 along the beam propagation direction to alter the focusing ofthe beam on the screen 101.

In the example in FIG. 18, the focus sensing mark 3310 includes multiplevertical stripe marks 3311 parallel to the fluorescent stripes andarranged in a periodic array along the horizontal scan direction. Thestripe width and the spacing between two adjacent stripes are equal tothe desired spot width. In operation, the laser beam is turned oncontinuously in a continuous wave (CW) mode while passing over the focussensing mark 3310. A detector monitors the reflected (or transmitted)light from the focus sensing mark 3310 as the laser beam scans acrossthe focus sensing mark 3310. If the beam spot size is of the desiredsize, the intensity of the feedback light generated by the focus sensingmark 3310 is a sine wave with 100% modulation as shown by the trace3302. When the beam spot size is greater than the desired in size, themodulation depth decreases. Hence, measuring the modulation of thesignal can be used to infer the spot size and to control the focusing ofthe laser beam. In some implementations, a second focus sensing mark canbe placed on the screen 101 at a different depth (optical path lengthfrom laser to screen). The signal modulations measured from the twofocus sensing mark can be compared to determine which way the focusingshould be adjusted to reduce beam size.

A power sensing mark may also be provided in the peripheral referencemark region on the screen 101 to direct a portion of the scanningexcitation beam 120 into a detector to monitor the laser power. Thisfeature can be used to monitor the laser power dynamically duringoperation. FIG. 19 shows a wide vertical stripe parallel to thefluorescent stripes as the power sensing mark 3410. FIG. 19 furthershows other marks in the peripheral reference mark region 2610. Otherreference marks are also shown in the region 2610. In operation, thelaser is turned on in a CW mode with a predetermined drive current priorto passing over the power sensing mark 3410. The driving currents of thelaser can varied when measuring the laser power in different scan linesto allow real time mapping of the power-current curve of the laser. Thepower measurements obtained from multiple scan lines can be averaged toreduce noise in the detection.

One way to correct the horizontal misalignment in the above displaysystems in FIGS. 8 and 10 is to program the display processor in thelaser module 110 to delay the modulated image signal carried by themodulated laser beam 120 by one sub color pixel time slot if the greendetector has an output and red and blue detectors have no output or bytwo sub color pixel time slots if the blue detector has an output andred and green detectors have no output. This correction of a spatialalignment error by a time delay may be achieved digitally within thedisplay processor. No physical adjustment in the optical scanning andimaging units in the laser module 110 is needed. Alternatively, theoptical imaging units and the scanning units in the laser module 110 maybe adjusted to physically shift the position of the excitation beam 120on the screen 101 so that the laser position on the screen 101 isadjusted horizontally to the left or right by one sub pixel in responseto the error detected by the on-screen optical sensing unit 810. Theoptical alignment by physically adjusting the scanning laser beam 120and the electronic or digital alignment by controlling the timing ofoptical pulses can be combined to control the proper horizontalalignment.

A test pattern can be used to check the horizontal alignment in thedisplay systems in FIGS. 8 and 10. For example, a frame of one of thered, green and blue colors may be used as a test pattern to test thealignment. FIG. 20A shows a test pattern for the color pixel embeddedwith the detectors in FIGS. 8 and 9 and the corresponding outputs of thethree detectors PD1, PD2 and PD3 when the horizontal alignment is properwithout an error. This test pattern can also be used in the system inFIG. 10. FIGS. 20B, 20C and 20D show three different responses generatedby the three detectors PD1, PD2 and PD3 when there is a misalignment inthe horizontal direction. The detector responses are fed to the lasermodule 110 and are used to either use the time-delay technique or theadjustment of the beam imaging optics to correct the horizontalmisalignment.

In the above servo control examples in FIGS. 8, 9 and 10, the on-screenor off-screen optical sensing unit detects the individual coloredsignals. In various implementations, it may be convenient to usescattered or reflected light of the scanning excitation beam 120incident to the screen 101 to detect the alignment between theexcitation beam and the fluorescent stripes of the screen 101. The abovedescribed servo reference marks are peripheral servo reference markslocated outside the fluorescent area of the screen. The followingsections further describe pixel-level servo reference marks in thefluorescent area of the screen that are used to determine the locationof a beam relative to the center of an individual subpixel on thescreen.

The periodic structure of the fluorescent stripes or periodic featuresformed on the periodic structures of the fluorescent stripes can be usedas servo reference marks which scatter or reflect a portion of thescanning excitation beam 120 and the scattered or reflected light fromsuch servo reference marks is detected to measure the presence of themisalignment and the direction of the misalignment. A temporal variationin timing of optical pulses is superimposed onto the scanning excitationoptical beam 120 and the optical detection of the position of the beamon the screen is achieved by measuring the scattered or reflected lightof the scanning excitation beam 120 by the servo reference marks. Theinformation of the beam position on the screen 101 with respect to theperiodic servo reference marks is used to control the alignment of thebeam on the screen 101.

For example, a servo feedback control of a scanning beam display systemcan be implemented as follows. A beam of excitation light modulated withoptical pulses is projected onto on a screen with parallel fluorescentstripes and is scanned in a beam scanning direction perpendicular to thefluorescent stripes to excite each fluorescent stripe to emit visiblelight which forms images. A temporal variation, e.g., the periodictemporal variation, is applied to the timing of the optical pulses inthe beam of excitation light to advance or delay a spatial position ofeach optical pulse along the beam scanning direction on the screen. Thereflection of the beam of excitation light from the screen is detectedto produce a monitor signal which varies with a position of the beamrelative to each fluorescent stripe. The information in the monitorsignal is used to indicate a spatial offset of an optical pulse relativeto a center of an intended or targeted fluorescent stripe along the beamscanning direction perpendicular to the fluorescent stripes. Based onthe spatial offset, the timing of the optical pulses in the beam ofexcitation light is adjusted to reduce the spatial offset.

This servo feedback control may be implemented in various ways. Ascanning beam display system with this servo feedback control caninclude an optical module operable to produce a scanning beam ofexcitation light which carries optical pulses that are sequential intime and carry image information; a screen comprising parallelfluorescent stripes which absorb the excitation light and emit visiblelight to produce images carried by the scanning beam; an optical sensorpositioned to receive scattered or reflected excitation light by thescreen and to produce a monitor signal indicating a spatial alignment ofthe beam relative to the screen; and a feedback control unit incommunication with the optical sensor and operable to control theoptical module so as to adjust timing of the optical pulses carried bythe beam of excitation light in response to the monitor signal. In thissystem, the optical module can be used to create a temporal variation intiming of the optical pulses in the beam. The screen can includeperiodic spatial features that modify the portion of the scattered orreflected light of the excitation light received by the optical sensorin relation with the temporal variation in timing of the optical pulsesin the beam. The feedback control unit can adjust the timing of theoptical pulses in response to information in the monitor signal that iscaused by the modification by the screen in the received portion oflight by the optical sensor and temporal variation.

FIG. 21 shows one example of such a scanning display system with servofeedback control based on the scanning display system in FIG. 3. Thescanning display system in FIG. 4 can also be implemented with thisservo feedback control. In FIG. 21, the signal modulation controller 320superimposes a temporal variation on the timing of the optical pulses inthe excitation beam 120 in control the modulation of the excitation beam120. Periodic servo reference marks are provided on the screen 101 toproduce feedback light 1201 that is either the scattered or reflectedlight of the scanning excitation beam 120 caused by the servo referencemarks or fluorescent light emitted by the servo reference marks underthe optical excitation of the scanning excitation beam 120. An opticalservo sensor 1210 is provided, e.g., an off-screen optical detector, tocollect the feedback light 1201 from the screen 101. More than oneoptical servo sensor 1210 can be used. An optical servo sensor 1210 canbe located at a suitable location off the screen 101 to maximize thecollection of the feedback light from the screen 101, for example, alocation near the scan lens 560 in the system in FIG. 5. A collectionlens 1220 may be placed in front of the optical servo sensor 1210 tofacilitate collection of light. The output of the optical servo sensor1210 is used as servo feedback signal and is fed to the signalmodulation controller 320. The signal modulation controller 320processes the servo feedback signal to determine the position offset ofan optical pulse from a center of a fluorescent stripe and then adjustthe timing of optical pulses in the scanning excitation beam 120 toreduce the position offset.

The periodic servo reference marks on the screen 101 can be in variousconfigurations. Referring to FIGS. 2A, 2B and 2C, the stripe dividersbetween the fluorescent stripes can be used as the servo referencemarks. Each stripe divider may include an additional structure as aservo reference mark. FIGS. 22 and 23 illustrate two examples.

In FIG. 22, each stripe divider 1310 can include a reflective orfluorescent layer 1312 as the servo reference mark located on the sideof the fluorescent layer of the screen 101 that receives the scanningexcitation beam 120. The reflective servo reference mark 1312 reflectsthe excitation beam 120 more than the fluorescent stripe which absorbsthe excitation light. Hence, the reflected excitation light varies inpower as the scanning excitation beam 120 scans across a fluorescentstripe. A thin reflective stripe can be coated on the end facet of eachstripe divider 1310 as the servo reference mark 1312. An opticallyabsorbent layer 1314 may be formed on the facet of the each stripedivider 1310 that faces the viewer to improve the contrast of the image.The detection sensitivity of the optical servo sensor 1210 enhances whenthere is a large difference in the reflection of the excitation lightbetween the servo reference mark 1312 and the fluorescent stripes. Ahigher detection sensitivity can be achieved in the optical servo sensor1210 when the servo reference mark 1312 is made of a fluorescent layerthat emits fluorescent light under illumination by the scanningexcitation beam 120. The fluorescent material for the mark 1312 can bedifferent from the fluorescent stripes so that the fluorescent lightemitted by the mark 1312 is at a different wavelength from the emissionwavelengths of the fluorescent stripes. As an example, the mark 1312 canbe an IR fluorescent material where the emitted IR light is invisible tothe human eye and thus does not affect the image quality to the viewer.An optical bandpass filter can be placed in front of the optical servosenor 1210 to allow only the emitted fluorescent light by the marks 1312to enter the optical servo sensor 1210.

FIG. 23 shows another screen design where stripe dividers 1410 betweenfluorescent stripes are made of an optically reflective or fluorescentmaterial. As an option, the divider facet that faces the viewer side ofthe screen can be coated with a blackened absorptive layer 1420 toreduce any reflection towards the viewer side, e.g., less than 10% inreflection and greater than 80% in absorption from 400 nm to 650 nm.This feature can enhance the resolution and contrast of the screen.

FIG. 24 further shows an implementation of the screen design in FIG. 23.All dimensions are in microns and are exemplary. Stripe dividers 1500are formed over a dielectric layer 1501. Fluorescent materials foremitting red, green and blue colors are filled in between the stripedividers 1500 to form the fluorescent stripes. Various opticallyreflective materials can be used to form the stripe dividers 1500. Metalmaterials such as aluminum can be used to construct the dividers 1500 oras coating materials to form a coating on a surface or facet of eachdivider 1500 that needs to be reflective. In addition, a white paintmaterial can also be used to form the dividers 1500 to achieve a highreflectivity. For example, a white paint made of a TiO2-filled resin ora barium sulfate-filled resin can be formulated to achieve superiorreflective properties to metal coatings, especially when reflecting backinto a clear polymer. For example, the reflectivity of the white paintmaterial can be grater than 90% from 400 nm to 650 nm. The stripedividers 1500 can also be made to include a fluorescent materialemitting light at a wavelength different from the excitation light 120and the visible light emitted by the fluorescent stripes under theillumination of the same excitation light 120 to improve the signal tonoise ratio at the optical servo sensor 1210.

FIG. 25 shows timing of optical pulses and beam positions on the screen101 when the excitation beam 120 scans across the fluorescent stripesalong the horizontal direction. The excitation beam 120 is modulated asa train of optical pulses in the time domain. As an example, threeconsecutive laser pulses 1601, 1602 and 1603 in the excitation beam 120for illuminating three consecutive fluorescent stripes 1610, 1620 and1630 on the screen 101 are shown to be at times t1, t2 and t3 during ascan. Each fluorescent stripe in FIG. 25 can be a stripe of a particularfluorescent material with a designated color or a stripe of a uniformwhite phosphor layer in combination of a stripe color filter for thedesignated color. The excitation beam 120 is scanned in a raster formatto produce a raster image made up by small dots, known as pixels withdifferent colors. Each pixel is usually made up of three sub-pixels inthree different primary colors red (R), green (G), and blue (B). Thesub-pixels are patterned on the screen in the form of the fluorescentstripes. The laser beam 120 is scanned from left to right along thehorizontal scan direction, one line at a time, to form the image. As thebeam 120 travels from left to right in each scan, it should beaccurately modulated in the time domain in order to properly address thesub-pixels. Hence, a pulse of the scanning excitation laser beam 120 isturned on at the same time when the beam 120 reaches the correspondingsub-pixel and is turned off when the beam 120 departs from thecorresponding sub-pixel. As illustrated, when the excitation beam 120 isproperly aligned with respect to the screen 101 along the horizontalscan direction, the pulse 1601 is on when the beam 120 is scanned to thecenter of the fluorescent stripe 1610, the pulse 1602 is on when thebeam 120 is scanned to the center of the fluorescent stripe 1620 and thepulse 1603 is on when the beam 120 is scanned to the center of thefluorescent stripe 1630. Beam footprints 1621, 1622 and 1623 illustratesuch aligned beam positions in the fluorescent stripes 1610, 1620 and1630, respectively.

When there is misaligned along the horizontal scan direction, each pulseis on when the beam 120 is scanned to an off-center position in afluorescent stripe. Beam footprints 1631, 1632 and 1633 illustrate suchmisaligned beam positions in the fluorescent stripes 1610, 1620 and1630, respectively. Consider the fluorescent stripe 1610 where the pulseshould be on when the beam 120 is at the position 1621 and off when thebeam 120 is at the position 1631. If the pulse is on when the beam 120at the position 1631 rather then the intended position 1621, thefluorescent strip 1610 is under illuminated by the beam 120 and aportion of the adjacent fluorescent strip at a different color isilluminated by the beam 120, i.e., the laser is turned on during thetransition time when the beam 120 is crossing from one color sub-pixelto the next one. In other words, this misalignment occurs when the pulsemodulation in time in the beam 120 is not synchronized with thesub-pixels in space. Under this condition, the color control can beadversely affected because the pulse that is supposed to turn on oneparticular color sub-pixel now “spills” over to the next different colorpixel, either within the same color pixel or between two adjacent colorpixels, to cause mis-registration of the image and to degrade the colorpurity of the image.

Therefore, it is desirable to accurately control the timing of thepulses of the scanning laser beam 120, i.e., the times to turn on andoff optical pulses with respect to the laser position on the screen. Inorder to control the timing of the laser pulses in the scanning beam120, a servo method is used to measure the beam offset based on thereflected light from the back of the sub-pixel when the laser is turnedon. The signal strength of the reflected light varies with the relativeposition of the laser light at each sub-pixel when the laser is turnedon at the center or is turned on off-center of the sub-pixel. Reflectorsor reflective features at the edge of each sub-pixel are used as theservo reference marks to generate reflected light from each sub-pixel tomonitor the position of the scanning laser beam 120 at each sub-pixel.As illustrated in FIG. 25, the beam 120 is least reflected by the stripedividers 1600 when the pulse is turned on at the beam position 1621 inthe center of the fluorescent stripe 1610 and the reflection by thedividers 1600 increases when the pulse is tuned on at the beam position1631 off the center of the fluorescent stripe 1610. This difference inthe power level of the reflected excitation light when the beam 120 isturned on at different positions relative to the center of fluorescentstripe and the temporal variation in the timing of the optical pulses inthe beam 120 are used to measure misalignment along the scan direction.

FIGS. 26A, 26B and 26C illustrates the variation of the signal strengthof the reflected excitation light when the excitation beam 120 atdifferent positions in a subpixel. In FIG. 26A, the pulse is tuned onwhen the beam 120 is at the left side of the subpixel. A strongreflection R1 is detected at the optical servo sensor 1210 the system inFIG. 21. When the pulse is turned on when the beam 120 is at the centerof the subpixel shown in FIG. 26B, a relatively weak reflection R2 isdetected at the optical servo sensor 1210. When the pulse is turned onwhen the beam is at the right hand side of the center of the subpixel, ahigher reflection of the excitation light is detected again at theoptical servo sensor 1210.

In general, the power level of the reflected excitation light varieswith the position of the beam 120 in a subpixel when the pulse is on.FIG. 27 illustrates an example of the sub-pixel design and the opticalpower density of the reflected excitation light from the subpixel atvarious laser beam positions. In this example, the sub-pixel includes acentral fluorescent stripe 1801 filled with an appropriate fluorescentmaterial that emits light of a designated color for that sub-pixel underexcitation of the scanning laser beam 120. Two stripe dividers 1802 arelocated at two opposite sides of the fluorescent stripe 1801 as twoperipheral reflectors are to generate the reflected light. Theperipheral reflectors may be designed as diffractive structures,reflective structures which can either specularly reflect light ordiffuse light, a wavelength conversion material such as a phosphor thatemits light under the excitation of the beam 120, or a mixture of bothdiffractive and reflective structures.

FIG. 28 illustrates an example of the relation of the reflected powerlevel and the beam position offset the center of a sub-pixel. Thesubpixel can be designed to produce a highest reflectivity R3 at theoutmost edge position from a center of the subpixel, a lowestreflectivity R1 at the center position of the subpixel and anintermediate reflectivity R2 for beam positions in between. As anexample, the highest reflectivity R3 can be generated when the beam 120is turned on at the center of a stripe divider. Hence, the level of thereflected light can be used to indicate the relative offset of the beamposition from the center of each subpixel.

The servo reference marks associated with the fluorescent stripesdescribed above allows the feedback light, either reflected light orfluorescent light, to vary in power with the position of the laser beamposition in each subpixel. This power variation in the feedback lightcan be used to determine whether the beam 120 is turned on at the centerof a subpixel or off the center of the subpixel. However, this powervariation does not provide information on the direction of the offset inthe position of the beam 120 in a subpixel. In order to produce a signof a servo signal to indicate the direction of the offset of theposition of the beam 120 in the subpixel, the scanning laser beam 120 isfurther modulated with a small delay signal superimposed on top of thescanning time of the laser beam 120 to control the timing of the opticalpulses in the beam 120. This delay signal produces a signal pattern inthe reflected light from the sub-pixels to indicate whether the positionof the laser pulse on the screen should be moved to the right or to theleft relative to the center of a sub-pixel or, in the time domain, thetiming of a laser pulse should be delayed or advanced. This delay signalis a periodic signal and, as the laser beam 120 scans the screen 101, ispositively and negatively delayed in a periodic fashion in the timedomain. This periodic variation in timing of the pulses can be, forexample, a sinusoidal wave or square wave.

FIG. 29 shows one example of this periodic delay signal in a sinusoidalform. In each period in time of the delay signal, the scanning beam 120sans over multiple subpixels along the horizontal scanning directionperpendicular to the fluorescent stripes. In the illustrated example, atotal of three subpixels are scanned by the scanning beam 120 in oneperiod of the delay signal. As the timing of the pulse is periodicallymodulated during the scan, the reflected signal detected by the opticalservo sensor 1210 in FIG. 21 varies, also in a periodic form.

FIG. 30 further illustrates, in time domain, how the pulse timing in thebeam 120 is modulated by the delay signal. Notably, the temporalvariation in timing of the optical pulses is set to correspond to aspatial shift in the beam position less than the width of thefluorescent stripes along the horizontal scanning direction. Hence, thesuperimposed delay signal provides a perturbation of the beam positionaround the current beam position within a fluorescent stripe or subpixelalong the horizontal beam scanning direction to cause a change in theservo signal. Because of the presence of the servo reference marks onthe stripe dividers, this change in the servo signal indicates thedirection and amount of the offset in the beam position from the centerof a fluorescent stripe or subpixel along the horizontal beam scanningdirection.

When the pulse is on as the beam 120 is at the center of a subpixel, thereflected light is at the minimum power level R1. When the pulse is onat other off-center positions, the reflected light has a higher powerlevel that varies with the amount of the offset from the center. In aperfectly aligned system, when the delay is equal to zero, the laserbeam “on time” is at the center of the sub-pixel. Under this condition,a reflected signal R1 is produced when the delay is positive ornegative. When the laser beam 120 is offset with respect to the centerof the sub-pixel due to the delay signal, the pulse of the beam 120 isturned on at offset positions near the center of the subpixel and thusthe reflected signal R2 is produced. Notably, under this condition, theperiod T1 of the oscillation in the reflected signal is one half of theperiod T0 of the delay signal.

FIG. 31 illustrates the beam position offset to the left of the centerof a sub-pixel that produces a highest reflectivity R3 at the outmostedge position from a center of the subpixel, a lowest reflectivity R1 atthe center position of the subpixel and an intermediate reflectivity R2for beam positions in between. Hence, when the laser beam 120 is turnedon at a position offset from the center of the sub-pixel to the left,the delay signal superimposed on top of the scanning laser beam variesthe positions of the laser beam 120 mostly to the left of the center ofeach sub-pixel. Hence, the reflected signal various in amplitude betweenthe levels R1 and R3 and is synchronized in phase with the delay signal.As such, the period T2 of the reflected signal is equal to the period T0of the delay signal. This state of the reflected signal indicates thatthe pulse is turned too early in time.

FIG. 32 illustrates an example for the beam position offset to the rightof the center of a sub-pixel. Under this condition, the delay signalsuperimposed on top of the scanning laser beam is such that the laserbeam is mostly to the right of the sub-pixels and the reflected signalis not synchronized with the delay signal is out of phase with the sameperiod as the delay signal (T3=T0). The amplitude of the reflectedsignal varies between the signal levels R1 and R3. This state of thereflected signal indicates the pulse in the laser beam 120 is turned ontoo late in time. Notably, the phase of the reflected signals relativeto the delay signals in FIGS. 31 and 32 are opposite and this differencecan be measured to determine the direction of the offset for the servocontrol.

FIG. 33 illustrates a method of using the detected reflected signal todetermine the direction of the beam offset from the center of thesubpixel. An offset indicator signal is defined as the integration ofthe products of the delay signal and the reflected signal at all timelocations over one delay period in the delay signal. The right hand sideof FIG. 33 shows an example of the offset indicator signal. A positivevalue of this signal indicates that the beam is turned on too early intime and is located to the left of the center of the subpixel. Anegative value of this signal indicates the beam is turned on too latein time and is located to the right of the center of the subpixel

In the centered case, the reflectivity signal has twice the oscillationfrequency of the delay signal. Hence an integration of the reflectivitysignal over one delay cycle results in a negligible servo error signal.The servo response circuit can be configured to maintain the currenttiming of the pulses without altering the position of the laser on time.When the laser is mostly off the center of the sub-pixel to the right orleft side, the reflected signal is out of phase with each other respectto the delay signal and each reflected signal has the same oscillationfrequency of the delay signal. The integration of an entire reflectivitycycle multiplied by the delay signal yields a positive or negative servoerror signal. In these two cases, the servo control mechanism can adjustthe timing of the pulses in the laser beam 120 to reduce the beam offsetand to achieve proper sub-pixel registration.

The change in the reflected signal is captured using an optical servosensor 1210 as shown in FIG. 21. One detection scheme is to use a widearea detector as the sensor 1210 to capture a portion of theback-scattered light. Improved SNR can be obtained with a lens (or otherflux collecting element such as a non-imaging concentrator). The lensrepresents a larger collecting area which brings more scattered light tothe detector. If the scattered light is of a different wavelength ascompared to the incident beam, e.g., when fluorescent servo referencemarks are used, then a spectral filter may be used to reject othersources of radiation (including any unwanted backscatter of the incidentbeam). In addition, multiple detectors may be placed in severallocations to improve detectability of the backscattered radiation.Because the signal may be weak compared to other sources of light, theservo signals may be averaged over many lines and frames to improve thesignal to noise ratio.

The delay signal can be either periodic or non-periodic with variousamounts of delay and periodicity. The concept of introducing a variabledelay signal is needed to figure out the directionality of thecorrection needed. Note that the delay signal is small enough so that itdoes not add color distortion to the screen. We assume that less than10% color bleed may result due to the delay signal. In someimplementations, the phase of the period delay on the laser beam can beshifted by 90 degrees from one scanning line to the next to reduce afixed pattern effect caused by the delay signal to a viewer.

In implementations, the servo signals from higher brightness areas ofthe screen can be measured and the amplitudes of the measured signalsare normalized by the amplitude of the outgoing video signal incontrolling the beam alignment along the horizontal scan direction. Thistechnique can improve the signal to noise ratio in the detection becausethe servo delay signal is superimposed on a variable amplitude videosignal.

The above time-delayed servo technique provides one approach tomitigation of the timing issue in systems where one beam is used todeliver the different colors on a display in order to accurately targetthe color elements. For screens where phosphors are arranged as parallelvertical stripes, the excitation laser beam is used to activatephosphors of the three primary colors, and as the beam scans across thephosphors, the beam activates each color sequentially in time. Thetargeting issue in space thus becomes a timing issue in controllingtiming of the laser pulses. The variations of the system components dueto temperature, aging and other factors and the component and devicetolerances during the manufacturing thereof need to be accounted for thetiming control of the laser beam on the screen. For example, thermalexpansion effects, and distortions in the optical imaging will needcorresponding adjustments in the precise timing to activate each colorin a pixel. If the laser actuation does not properly correspond to thetiming where the beam is directed with the central portion of asub-pixel and is crossing the intended phosphor, the beam will eitherpartially or completely activate the wrong color phosphor.

In addition to the servo control, a calibration “map” of timingadjustments can be provided to assist the servo control for correctingthe timing over different portions of the screen. This calibration mapincludes beam alignment data for all sub-pixels on the screen and can beobtained using the servo control to measure alignment of the entirescreen after the assembly of the display system is completed at thefactory. This map of adjustments can be stored in the memory of thelaser module 110 and reused for an interval of time if the effects thatare being compensated for do not change rapidly. In operation, when thedisplay system is turned on, the display system can be configured to, asa default, set the timing of the laser pulses of the scanning laser beambased on the alignment data in the calibration map and the servo controlcan operate to provide the real-time monitoring and control of the pulsetiming during the operation. Additional calibration measurements may bemade to update the stored calibration map in the memory. For example, asingle or multiple consecutive versions of this map could be placed inthe same memory that is used for buffering pixel color data. Thesecalibration maps may be encoded to reduce both the amount of memory theyoccupy and the bandwidth of memory needed to access them. For the caseof smoothly changing timing adjustments, a simple scheme such as deltamodulation can be used effectively to compress these maps.

The calibration “map” can be obtained by operating each scanning laserbeam 120 in a continuous wave (CW) mode for one frame during which thescanning laser beams simultaneously scan through the entire screen, onesegment at a time, when multiple lasers are used as shown in FIG. 5. Ifa single laser is used, the single scanning beam is set in the CW modeto scan the entire screen, one line at a time. The feedback light fromthe servo reference marks on the stripe dividers is used to measure thelaser position on the screen. The monitor signal from the photo detectorcan be sent through an electronic “peak” detector that creates a pulsewhenever the monitor signal is at its highest relative amplitude. Thetime between these pulses can be measured by a sampling clock. Becausethe scan speed of the scanning beam on the screen is known, the timebetween two adjacent pulses can be used to determine the spacing of thetwo locations that produce the two adjacent pulses. This spacing can beused to determine the subpixel width and subpixel position. Depending onthe beam scan rate and the frequency of the sampling clock, there aresome nominal number of clocks for each sub-pixel. Due to opticaldistortions, screen defects or combination of these the distortions anddefects, the number of clocks between two adjacent pulses for any givensub-pixel may vary from the nominal number of clocks. This delta can beencoded and stored in memory for each sub-pixel.

FIG. 34 shows one example of the detected reflected feedback light as afunction of the scan time for a portion of one horizontal scan, therespective output of the peak detector and the sampling clock signal. Anominal subpixel and an adjacent short subpixel are illustrated. FIG. 35shows one example of the detected reflected feedback light as a functionof the scan time for a portion of one horizontal scan, the respectiveoutput of the peak detector and the sampling clock signal where anominal subpixel and an adjacent long subpixel are illustrated.

During calibration, contaminants such as dust on the screen, screendefects, or some other factors may cause missing of an optical pulse inthe reflected feedback light that would have been generated by a servoreference mark between two adjacent subpixels on the screen. FIG. 36illustrates an example where a pulse is missing. A missing pulse can bedetermined if a pulse is not sampled within the nominal plus the maximumexpected deviation from nominal number of clocks. If a pulse is missed,the nominal value of clocks can be assumed for that sub-pixel and thenext sub-pixel will contain the timing correction for both sub-pixels.The timing correction can be averaged over both sub-pixels to improvethe detection accuracy. This method may be extended for any number ofconsecutive missed pulses.

A scanning beam display system can be implemented using various featuresdescribed above. For example, such a system can include an opticalmodule operable to produce a scanning beam of excitation light havingoptical pulses that are sequential in time and carry image information,and a fluorescent screen that includes a fluorescent area and aperipheral servo reference mark area outside the fluorescent area. Thefluorescent area absorbs the excitation light and emits visiblefluorescent light to produce images carried by the scanning beam. Thefluorescent area includes first servo reference marks producing a firstfeedback optical signal under illumination of the scanning beam. Theperipheral servo reference mark area includes at least one second servoreference mark producing a second feedback optical signal underillumination of the scanning beam. This example system includes twoseparate sensors for the servo control: (1) a first optical sensorpositioned to receive the first feedback optical signal and to produce afirst monitor signal indicating a spatial alignment of the opticalpulses on the fluorescent screen; and (2) a second optical sensorpositioned to receive the second feedback optical signal and to producea second monitor signal indicating an optical property of the scanningbeam on the fluorescent screen. A feedback control unit is included inthe optical module to adjust the scanning beam in response to the firstand second monitor signals to control at least the spatial alignment ofspatial positions of the optical pulses on the fluorescent screen.

The above second optical sensor for detecting the second feedbackoptical signal from the peripheral servo reference mark area on thescreen can be an optical detector that is connected to a light pipe thatis connected to the peripheral servo reference mark area on the screen.In one implementation, the second servo reference mark in the peripheralservo reference mark area can be transmissive so that the transmittedlight through the mark when illustrated by the excitation beam 120 iscoupled into one end of the light pipe that is connected to the otherside of the mark, e.g., on the viewer side of the screen. The light pipecan be a channel with reflective surfaces formed by dielectricinterfaces under the total internal reflection (TIR) condition ormetallic reflective side wall surfaces. The second optical sensor can belocated at the other end of the light pipe to receive the light signalguided by the light pipe. When different types of servo reference marksare provided in the peripheral servo reference mark area for monitoringdifferent parameters, e.g., beam focusing and beam SOL position,different light pipes can be implemented in the peripheral servoreference mark area for different reference marks. Each light pipedirects the signal to its respective optical detector.

As described above, different facets of a polygon scanner tend to havedifferent facet orientations with respect to the scanner rotation axisdue to the inaccuracy in manufacturing and other factors and suchpyramidal errors can degrade the performance of the displayed images.FIG. 5 shows a scanning display system with a fluorescent screen whereone or more excitation laser beams 532 are scanned by a galvo mirror 540and a polygon scanner 550 onto a fluorescent screen 101. A displaysystem using a passive screen can also be constructed based on thedesign in FIG. 5 where three modulated beams in different colors (e.g.,red, green and blue) carrying image data can be overlapped as a singlebeam that is scanned by the galvo mirror 540 and the polygon scanner 550on a passive screen to form colored images.

FIG. 37 illustrates the operation of the galvo mirror 540. Each cycle ofmotion of the galvo mirror 540 is divided into a regular scanning phaseand a vertical retrace phase. In the regular scanning phase, the galvomirror 540 scans over a scan time during which the light is directed tothe galvo mirror 540 and the galvo mirror 540 scans the light to producemultiple horizontal scan lines at different vertical positions. Duringthis scan time, multiple facets of the polygon 540 scan the beam. In thevertical retrace phase, the light is turned off or blocked and the galvomirror 540 resets its position back to the initial scanning positionfrom which another vertical scan begins. Depending on the design of thegalvo mirror 540, the vertical retrace time usually takes a smallfraction of the total scan, e.g., 5%. As an example, if the verticalscanning rate is 60 Hz which corresponds to a vertical scan time ofabout 16.7 ms, the vertical retrace time is 5% ( 1/60)=890 microseconds.

FIG. 38 illustrates the effect of the pyramidal error of the horizontalpolygon scanner 550 in FIG. 5. Two adjacent facets, facet No. 1 andfacet No. 2, have different facet orientations with respect to thevertical rotation axis of the scanner. Hence, for the same incident beamto be scanned, the facets No. 1 and No. 2 project the beam at twodifferent vertical locations on the screen separated by a distance D onthe screen. If the polygon 550 is away from the screen by a distance L,the pyramidal error is ½ tan (D/L).

FIGS. 39A and 39B further illustrate the effect of the pyramidal errorsin the polygon scanner 550 in FIG. 2 when multiple scanning beams arescanned at the same time as shown in FIG. 7. If the polygon 550 is freeof pyramidal errors, different beams are simultaneously scanned at theirdesignated vertical locations to produce parallel horizontal scan lineswith an even vertical spacing. Due to the vertical scanning, eachhorizontal line is not perfectly horizontal and is skewed (FIG. 39A).If, however, the polygon 550 has pyramidal errors, different beams aresimultaneously scanned to produce parallel horizontal scan lines withuneven vertical spacing and some of the horizontal lines depart fromtheir ideal vertical position (FIG. 39B).

One exemplary technique to mitigate the effect of the pyramidal error isto design the scanning system so that each horizontal scanning line insuccessive frames is scanned by different facets on the polygon. Hence,each line perceived by the viewer appears to be thicker with a linewidth along the vertical direction about the position spread on the samehorizontal line caused by the pyramidal errors. This techniqueessentially averages out the pyramidal errors and at the same timeslightly degrades the vertical resolution on the screen.

In operation, this technique scans each horizontal line by using adifferent polygon facet in successive frames to cause the same line atsuccessive frames to be at different vertical positions to cause a“blurred” line due to different pyramidal errors on different polygonfacets. As one specific example, the retrace time on the galvo mirror540 can be set to be less than 1 facet time. The assignment of thefacets for scanning different lines can be made in variousconfigurations. For example, the facets designated for retracing line 1can be 5, 6, 7, 8, or 9 facets in the following arrangement:

# of FACETS FACET ASSIGNMENT FOR SCANNING LINE 1 5 1 2 3 4 5 1 6 1 2 3 45 6 1 7 1 4 7 3 6 2 5 1 8 1 8 7 6 5 4 3 2 1 9 1 5 9 4 8 3 7 2 6This facet assignment for scanning can be achieved by controlling thepolygon scanner.

The above technique is an averaging technique and the averaging can alsobe implemented by dithering the vertical scanning mirror with a smallamplitude at a dither frequency higher than the frame rate. Thisdithering is controlled so that the vertical spread of the scanning beamon the screen caused by the dithering is the beam position spread on thesame horizontal line caused by the pyramidal errors. FIG. 40 shows theeffect of the high-frequency dithering of the galvo mirror 540. Severaldithering options may be implemented: a constant dither amplitude, avarying dither amplitude, a constant dither frequency, a varying ditherfrequency, or a combination of any of the above. The distribution of thedither (amplitudes, and or, frequencies) can be random, Gaussian orother profiles such as white noise, pink noise where the amplitude of arandom signal decreases as the signal frequency increases whilemaintaining constant signal power per frequency increment.

In addition, the light intensity of the scanning beam may be adjusted byreducing the intensity at a position where the pyramidal error causesthe horizontal lines to be denser and increasing the intensity at aposition where the pyramidal error causes the horizontal lines to besparser. This adjustment of light intensity can reduce the effect of thepyramidal error on the image quality.

Hence, based on the above, a scanning beam display system can beconfigured to include a polygon scanner having reflector facets andoperable to rotate to scan an optical beam along a first direction(e.g., the horizontal direction) and a second scanner having a reflectorto cause the optical beam to scan in a second direction (e.g., thevertical direction) perpendicular to the first direction. This systemcan include a control unit in communication with the second scanner tocontrol scanning of the second scanner. The control unit can be operableto dither the second scanner to cause the optical beam to change itsdirection back and forth along the second direction during each scan ata dither frequency higher than a frame rate of an image carried by theoptical beam. In one implementation, this system can include a mechanismto control a light intensity of the optical beam in a relation withpyramidal errors of different facets in the polygon scanner.

Also, based the above, a method for display can be provided to operate ascanning beam display. In this method, a polygon scanner havingreflector facets is used to scan an optical beam along a first directionand a second scanner having a reflector is used to scan the optical beamin a second direction perpendicular to the first direction. The scanningof the optical beam is controlled to scan the optical beam withdifferent facets of the polygon scanner at each horizontal scanning linein successive frames. In one implementation of this method, the secondscanner can be dither to cause the optical beam to change its directionback and forth along the second direction during each scanning at adither frequency higher than a frame rate of an image carried by theoptical beam. In addition, the light intensity of the optical beam canbe controlled in a relation with pyramidal errors of different facets inthe polygon scanner to reduce the any visual effect of the pyramidalerrors on the screen.

Referring to FIGS. 13-16, a fluorescent screen can have one or moreperipheral reference mark regions outside the display region to providevertical reference marks that measure the vertical position of ascanning beam. The screen design with one or more peripheral referencemark regions and vertical reference marks or other marks can be extendedto passive screens without fluorescent materials. A detection circuitcan be used to extract the vertical position information of the beamfrom the optical signals obtained from the reference marks. As describedabove, a feedback control can be applied to the galvo mirror 540 tocorrect a vertical shift caused by a pyramidal error of a facet in thepolygon scanner 550. Because the pyramidal error can vary from facet tofacet, the incident laser beams are not uniformly reflected to thedisplay in the vertical dimension on each horizontal sweep.

FIGS. 41, 42 and 43 show an exemplary implementation of a servo-basedpyramidal correction using the vertical scanner and on-screen verticalreference marks for scanning display systems with either passive oractive screens.

In this example, the scanning display has six lasers to direct six laserbeams to the screen. Referring to FIG. 7, the six laser beams arescanned at the same time to over one segment of the screen at a time andthen sequentially to scan other segments sequentially arranged in thevertical direction of the screen. One polygon facet scans all six laserbeams along the horizontal direction to cover one vertical screensegment at a time and different polygon facets sequentially scandifferent vertical segments.

The screen in this system is designed to include vertical referencemarks in each of the multiple vertical screen segments for monitoringthe pyramidal error of the polygon scanner. FIG. 41 shows four exemplarysuccessive horizontal scans labeled as Scan#1, Scan#2, Scan#3 and Scan#4over four successive vertical screen segments, respectively, by foursuccessive facets on the polygon scanner. Each scan has six laser beamsfrom six lasers labeled as laser#1, laser#2, laser#3, laser#4, laser#5and laser#6. The screen includes a peripheral reference mark region 4121where displayed images do not appear and a central display region 4120where the displayed images appear. In the peripheral reference markregion 4121, vertical reference marks 4110 are located in the upper leftcorner of the first vertical screen segment to measure the verticallocation of the first laser beam from the laser#1. Similarly, verticalreference marks 4120, 4130 and 4140 are located in the upper left cornerof the second, third and fourth vertical screen segments, respectively,to measure the vertical position of laser#1. Alternatively, the verticalreference marks in each screen segment can be located at a different andknown location to measure the vertical position of the laser beam fromthe laser#1 or another laser beam. The vertical reference marks in eachscreen segment are used to measure a vertical position of one of the sixlaser beams and this measured vertical location is used to represent thevertical position of that laser scan reflected from a correspondingpolygon facet. The vertical reference marks in each screen segment andthe detection of the vertical position can be implemented by theexamples in FIGS. 13-16.

When the polygon scanner is free of any pyramidal error, the laser scans#1 through #4 should be evenly spaced from one scan to another. Thevertical position signals from the different vertical reference marks indifferent screen segments should either indicate an identical offset inthe vertical position for each polygon facet, or no error in thevertical position of all scans. When the polygon scanner has a pyramidalerror in at least one facet, the error signals are different. Theposition of the vertical scanner can be controlled to either equalizethe errors in the vertical position or minimize the error in thevertical position to correct the effect of a pyramidal error on thescreen.

In the illustrated four successive scans on the screen from foursuccessive polygon facets, scan #1 and scan #2 are aligned verticallywith their respective vertical reference marks and thus do not showpyramidal errors from the corresponding facets on the polygon scanner.However, the facets for producing the scan#3 and scan#4 show pyramidalerrors: scan #2 and scan #3 are too close to each other because thescan#3 is too high in its vertical position, and scan #3 and #4 are toofar apart because the scan#3 is too high vertically and the scan#4 istoo low vertically. This is due to pyramidal error displacing the beamfrom facet to facet.

FIG. 42 illustrates operation of the vertical reference marks in eachscreen segment shown in FIG. 41 based on the reference mark in FIG. 13.Each reference mark includes two symmetric triangular reference marks2811 and 2812 that are displaced from each other. In operation, laserlight reflected from the triangular features 2811 and 2812 on the screengenerates the waveform shown in the lower part of FIG. 42. The lightfrom the first triangle is integrated. The detector circuit can bedesigned to generate a pulse with a pulse width proportional to thelength of the scanning beam on each reference mark. The light from thesecond triangle reference mark 2812 is integrated, then compared to thevalue for the detector signal from the first triangle mark 2811. Whenthe beam is vertically centered between the two triangle reference marks2811 and 2812, subtracting the first integrated value from the secondshould result in zero. This error signal is passed to themicro-controller, which is used by the microcontroller to determine theamplitude and direction of the galvo position error, and to correct thecorresponding pyramidal error in a subsequent scan by the same facet.

In the example illustrated in FIG. 42, the beam position 4211 is toohigh and thus generates a detector signal 4210. The difference betweenthe pulses from the two reference marks is negative and indicates thebeam's vertical position is too high. Accordingly, the next time thesame facet performs a horizontal scan, the vertical scanner position isadjusted to offset this measured pyramidal error. As another example,the beam position 4221 is proper and generates two equal pulses in thedetector signal 4220. The corresponding error signal is zero and nocorrection is needed for that facet. As yet another example, the beamposition 4231 is too low and thus generates a detector signal 4230. Thedifference between the pulses from the two reference marks is positiveand indicates the beam's vertical position is too low. Accordingly, thenext time the same facet performs a horizontal scan, the verticalscanner is adjusted to offset this measured pyramidal error.

FIG. 43 shows a block diagram of the vertical scanner control. Thescreen 4301 has a peripheral reference region with vertical referencemarks and a central display region for display images. An opticaldetector 4310 is at a location in front of the screen 4301 to receivelight from the vertical reference marks and to produce a detectorsignal. A pyramidal error signal generator circuit 4312 is used toreceive and process the detector signal to produce the error signal. Forexample, the circuit in FIG. 14A can be used to construct this circuit4312. A control 4314 uses the error signal to produce a galvo controlsignal that controls the vertical position of the galvo mirror 540 tocorrect the detected pyramidal error of a respective polygon facet. Thecontrol 4314 may include a microprocessor or microcontroller whichgenerates the control signal. In operation, the microcontroller uses theerror signal described above to slightly re-position the galvo mirror540 from its normal ramp position. This re-positioning corrects thesmall vertical deflection error of the laser beams caused by thepyramidal polygon error.

In a single frame, the laser beams are deflected from top to bottom ofthe screen 4301 in a continuous manner by a rotary motion of the galvomirror 540. Ideally, the galvo mirror 540 scans in a continuous, linearangle to direct light from top to bottom of the screen 4301. However,during this vertical scanning, different successive polygon facets areinvolved in the horizontal scanning and thus a polygon pyramidal errorfor any of the horizontal scans need to be corrected by a small offsetof the galvo mirror 540 at each respective horizontal scan in the middleof otherwise continuous vertical scanning by the galvo mirror 540. Toavoid any visible effect on the screen, the small offset to the verticalpositioning of the galvo mirror 540 is applied when the beam is in theperipheral reference mark region and outside the central display regionof the screen 4301.

FIG. 44 illustrates the operation of the galvo mirror 540 in correctingpyramidal errors during the scanning for a single video frame on thescreen 4301. In this example, a polygon scanner with eight facets isused as the scanner 550 and the screen 4301 is divided into sixteenvertical screen segments so that the polygon scanner 550 rotates tworevolutions to scan through the entire screen and to produce a singlevideo frame. As illustrated, each correction to the galvo angle ischanged slightly between the end of one horizontal and the start of asubsequent new horizontal scan. During this transition time between thetwo successive horizontal scans, the laser beam is not viewable on thedisplay. In the example, four pyramidal corrections are applied duringone revolution of the polygon: a first correction at the beginning ofthe horizontal scan by the second facet which also applies to the nextscan by the third facet, a second correction at the beginning of thehorizontal scan by the fourth facet which continues to the fifth facet,and a third correction at the beginning of the horizontal scan by thesixth facet which continues to the seventh facet, and a fourthcorrection at the beginning of the eighth facet. The pattern for thecorrection has changes that are usually relatively small and form amicrojog pattern. The same corrections are repeated for each revolution.In this example, the galvo retrace is done in a time for scanningthrough three polygon facets. The pattern in this example is slipped bythree facet times, and the next video frame scan begins on facet #4. Themicrojog pattern repeats for each rotation of the polygon, and not foreach video frame.

The microjog timing can be performed by the micro-controller in thecontrol 4314 in FIG. 43, and the angle is increased or decreased by theamplitude and direction determined from the error signal. The polygon550 can be configured to include a facet number sensor which providesfacet ID numbers for the facets. The micro-controller receives the facetID numbers and knows which facet on the polygon will be the next facetto direct the laser beams on the screen, and will control the galvomirror 540 according to the sequence of the facets for the horizontalscanning.

The above pyramidal correction includes an error measurement mechanismbased on vertical reference marks on the screen and optical detection ofscattered or reflected light from the vertical reference marks tomeasure a pyramidal error of each polygon facet and an error correctionmechanism that controls the vertical scanner in subsequent horizontalscans to correct the measured pyramidal error. Hence, as the pyramidalerror changes, the error measurement mechanism can detect the change andthus can adjust the correction to the vertical scanner accordingly. Thisdynamic nature of the pyramidal correction can be used to enhance thedisplay performance and to improve the reliability.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis specification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination. Forexample, based on the screen designs described above, a screen may bestructured to include the first dichroic layer D1, the fluorescent layerand the contrast enhancement layer without the second dichroic layer D2.In another example, a screen may include a lenticular layer or the lensarray layer with an array of parallel cylindrical lenses, and afluorescent layer with parallel fluorescent stripes that respectivelyare aligned with the cylindrical lenses. Hence, screens with variousstructures may be formed based on various layer designs described inthis application to meet specific considerations in applications.

Only a few implementations are disclosed. However, it is understood thatvariations and enhancements may be made.

1. A scanning beam display system, comprising: an optical modulecomprising a first scanner to scan along a first direction at least onescanning beam having optical pulses that are sequential in time andcarry image information, and a second scanner having a polygon with aplurality of reflective facets, the polygon operable to rotate around arotation axis that is along the first direction to scan the at least onescanning beam along a second direction perpendicular to the firstdirection; a screen positioned to receive the at least one scanning beamfrom the optical module and configured to include (1) a display regionwhich displays images carried by the at least one scanning beam, and (2)reference marks positioned in paths along the second direction of theleast one scanning beam on the screen and displaced from one anotheralong the first direction, each reference mark operable to produce, inresponse to the at least one scanning beam, an optical monitor signal oflight, and to direct the optical monitor signal of light to propagate inspace away from the screen, the optical monitor signal of lightcontaining information on a position offset of the least one scanningbeam relative to the reference mark along the first direction on thescreen when illuminated by the at least one scanning beam; an opticaldetector positioned to receive light of the optical monitor signals fromthe reference marks on the screen that propagate through space betweenthe screen and the optical detector and to produce respective electricaldetector signals, each detector signal containing the information on aposition offset of the least one scanning beam relative to a respectivereference mark along the first direction on the screen; and a firstscanner control that measures a pyramidal error of the polygon from eachdetector signal and controls scanning of the first scanner to correctthe position offset caused by the pyramidal error.
 2. The system as inclaim 1, wherein: the screen comprises parallel fluorescent stripes inthe display region which absorb light of the at least scanning one beamto emit fluorescent light and to produce the images carried by the atleast one scanning beam, and wherein the reference marks are locatedoutside the display region.
 3. The system as in claim 2, wherein: eachreference mark is optically reflective.
 4. The system as in claim 2,wherein: each reference mark is optically fluorescent to emit light ofthe optical monitor signal under illumination by the at least onescanning beam.
 5. The system as in claim 4, wherein: the optical monitorsignal is at a wavelength different from the fluorescent light emittedby the screen.
 6. The system as in claim 1, wherein: the display regionof the screen is free of a light-emitting fluorescent material andoperates to use light of the at least one scanning beam to present theimages carried by the at least one scanning beam.
 7. The system as inclaim 1, wherein: each reference mark is optically reflective.
 8. Thesystem as in claim 1, wherein: each reference mark is opticallyfluorescent to emit light of the optical monitor signal underillumination by the at least one scanning beam.
 9. The system as inclaim 1, wherein: each reference mark comprises first and secondfeatures separated from each other along the first direction and alongthe second direction.
 10. The system as in claim 9, wherein: the firstscanner control comprises an error signal generator that generates anerror signal from first and second signal components in the detectorsignal that are generated by the first and second features,respectively, to indicate the position offset of the least one scanningbeam relative to a respective reference mark on the screen.
 11. Thesystem as in claim 1, wherein: the first scanner control comprises amechanism that identifies a facet of the polygon that generates themeasured pyramidal error and controls the scanning of the second scannerto correct the position offset caused by the pyramidal error when theidentified facet subsequently scans the at least one scanning beam. 12.The system as in claim 1, wherein: the first scanner is a galvo mirror.13. A method for operating a scanning beam display system, comprising:using a first scanner to scan at least one beam of light modulated withoptical pulses to carry images along a first direction on a screen and asecond polygon scanner with a plurality of reflective facets to scan theat least one beam along a second, perpendicular direction on the screento display the images; using a plurality of reference marks on thescreen at positions that are respectively in beam scanning paths of theat least one beam at different positions along the first direction toproduce optical monitor signals of light when illuminated by the atleast one beam during scanning, and to direct the optical monitorsignals of light to propagate in space away from the screen, eachoptical monitor signal of light having information on a position offsetof the at least one beam relative to a respective reference mark alongthe first direction on the screen caused by a pyramidal error of arespective reflective facet in the polygon scanner; collecting light ofthe optical monitor signals from the reference marks on the screen toconvert collected light of the optical monitor signals into respectiveelectrical detector signals, each detector signal containing theinformation on the position offset corresponding to each detectedoptical monitor signal; and adjusting the scanning of the first scanneralong the first direction to reduce the position offset of the at leastone beam on the screen in response to the position offset in thedetector signal.
 14. The method as in claim 13, further comprising:controlling the scanning of the at least one beam to display the imagesin a central region of the screen; and making the adjusting of thescanning of the first scanner to reduce the position offset when the atleast one beam is outside the central region.
 15. The method as in claim14, wherein: the reference marks are located outside the central region.16. The method as in claim 13, further comprising: measuring theposition offset of the least one beam relative to a respective referencemark on the screen caused by a pyramidal error of a respectivereflective facet in the polygon scanner in a first scan of the at leastone beam along the second direction; and making the adjusting of thescanning of the first scanner to reduce the position offset in asubsequent scan by the respective reflective facet.
 17. A scanning beamdisplay system, comprising: an optical module operable to produce ascanning beam of excitation light having optical pulses that aresequential in time and carry image information; a first scanner to scanthe scanning beam along a first direction; a second scanner comprising apolygon having reflective facets and operable to spin around an axisparallel to the first direction and to use the reflective facets to scanthe scanning beam along a second, perpendicular direction; a fluorescentscreen comprising a fluorescent area having a plurality of parallelfluorescent stripes each along the first direction and spatiallydisplaced from one another along the second direction and a peripheralservo reference mark area outside the fluorescent area, whereinfluorescent stripes absorb the excitation light and emit visiblefluorescent light to produce images carried by the scanning beam, andthe fluorescent area comprises a plurality of first servo referencemarks producing a first feedback optical signal of light underillumination of the scanning beam to indicate a spatial alignment of theoptical pulses to the fluorescent stripes along the second direction andto direct the first feedback optical signal of light to propagate inspace away from the screen, wherein the peripheral servo reference markarea comprises a plurality of second servo reference marks eachproducing a second feedback optical signal of light under illuminationof the scanning beam indicating a position offset of the scanning beamalong the first direction and to direct the second feedback opticalsignals of light to propagate in space away from the screen; a firstoptical sensor positioned to receive light of the first feedback opticalsignal and to convert the received light of the first feedback opticalsignal into a first electrical monitor signal indicating the spatialalignment of the optical pulses relative to the fluorescent stripesalong the second direction; a second optical sensor positioned toreceive light of a second feedback optical signal and to convert thereceived light of the second feedback optical signal into a secondelectrical monitor signal indicating the position offset of the scanningbeam along the first direction when scanned by a respective reflectivefacet of the polygon; and a control unit operable to adjust the scanningbeam in response to the first and second monitor signals to control atleast the spatial alignment of spatial positions of the optical pulsesrelative to the fluorescent stripes and to reduce the position offset ofthe scanning beam along the first direction.
 18. The system as in claim17, wherein: each second servo reference mark is optically reflective.19. The system as in claim 17, wherein: each second servo reference markis optically fluorescent to emit light of the second feedback opticalsignal under illumination by the scanning beam.
 20. The system as inclaim 17, wherein: each second servo reference mark comprises first andsecond features separated from each other along the first direction andalong the second direction.
 21. The system as in claim 20, wherein: thecontrol unit comprises an error signal generator that generates an errorsignal from first and second signal components in the second feedbackoptical signal that are generated by the first and second features,respectively, to indicate the position offset of the at least onescanning beam relative to a respective reference mark on the screen. 22.The system as in claim 17, wherein: the control unit comprises amechanism that identifies a facet of the polygon that generates apyramidal error that causes a position offset and controls the scanningof the second scanner to correct the position offset caused by thepyramidal error when the identified facet scans the scanning beam. 23.The system as in claim 17, wherein: the optical module produces aplurality of scanning beams, the first and second scanners scan thescanning beams along the first and second directions on the screen, andthe second scanner scans the scanning beams simultaneously with a commonreflective facet along the second direction over one screen segment at atime and scans the scanning beams over the entire screen by sequentiallyscanning different screen segments at different times with differentreflective facets.
 24. The system as in claim 1, wherein: the firstscanner includes a dither mechanism which dithers the first scanner to,in addition to scanning the scanning beam along the first direction,dither the at least one scanning beam to change a beam direction backand forth along the first direction while scanning the scanning beamalong the first direction; and the first scanner control furtheroperates to control an amount of the dithering of the first scanner anda frequency of the dithering to be higher than a frame rate of an imagecarried by the scanning beam to reduce an effect of the pyramidal error.25. The system as in claim 24, comprising: a mechanism to control alight intensity of the optical beam in a relation with pyramidal errorsof different facets in the polygon scanner.
 26. The system as in claim17, wherein: the first scanner includes a dither mechanism which dithersthe first scanner to, in addition to scanning the scanning beam alongthe first direction, dither the scanning beam to change a beam directionback and forth along the first direction while scanning the scanningbeam along the first direction; and the control unit includes a firstscanner dither control mechanism that operates to control an amount ofthe dithering of the first scanner and a frequency of the dithering tobe higher than a frame rate of an image carried by the scanning beam toreduce an effect of the pyramidal error.
 27. The system as in claim 26,comprising: a mechanism to control a light intensity of the optical beamin a relation with pyramidal errors of different facets in the polygonscanner.
 28. The method as in claim 13, comprising: dithering the firstscanner, in addition to scanning the at least one beam in the firstdirection, to dither a beam direction back and forth along the firstdirection during each scanning along the first direction at a ditherfrequency higher than a frame rate of an image carried by the at leastone beam.
 29. The method as in claim 28, comprising: controlling a lightintensity of the at least one beam in a relation with pyramidal errorsof different reflective facets in the second polygon scanner.
 30. Themethod as in claim 13, comprising: using different facets of the secondpolygon scanner to scan the at least one beam to generate each scanningline in the second direction in successive frames, respectively, so thatthe same scanning line at a given position along the second direction insuccessive frames is scanned by different polygon reflective facets ofthe second polygon scanner onto the screen at respective positionsaround the given position determined by pyramidal errors of the polygonreflective facets.